This is something new I am trying out. A review on how others view a champion, my thoughts and opinion on his play-style , viability and how much pain can they induce in Alliance War Defense Nodes. If there any suggestions/criticisms, post it in the comments down below.
Now, on to the the main course -
Before we begin, I would say Kabam's Animation and Effects team hit the ball way out of the park with Man-Thing's animations. His particle animations are extremely smooth and at the same time a bit slow, which was what the game design team was going for (I think?). Out of all the three champions released this month, I think Man-Thing's the best animation wise. I would like hear what say about this though.
However, his abilities seem a bit iffy and I think the champion design team struggled to find a good balance for this character. Let's talk about that now.
A few notes before we move on (So that no one is out of the loop) -
M - Medium Attack
L - Light Attack
H - Heavy Attack
Sig. Level - Signature levels
DOT - Damage Over Time
All %s are displayed considering a 5* rank 3 with sig. level 200.
His design seems to be a "proper mystic" giant tank who deals damage via DOT effects (poison debuffs) with some utilities built-in.
He two main damage-dealing mechanics. One is his Toxic Pustules and the other being his Agitation mechanic. However, I would say maximizing his Toxic Pustules should be the priority while playing him and using the Agitation mechanic would be the secondary source of damage.
The best way of getting his Pustules is by throwing a Special 1 and gaining 3 of those buffs. Each of those buffs give a poison and an armor break while it expires. The poison from these deal around 93% of Man-Thing's base attack over 15 seconds. However, these poison debuffs damage have +50% potency when nullified via heavy attacks. So, in most cases, the poison damage is around 140% of Man-Thing's base attack, which is really neat. His Armor Break (-222.3 Amor Rating on opponent) lines up similar numbers to what Captain America (Infinity War) can do while parrying with a Cosmic Champion on team, so no that great for damage, but duration of 15 seconds makes it useful for utility purposes. Also, with 12-13 of these Armor breaks can let Man-Thing deal significant damage.
The secondary damage mechanic (Agitation) can provide upto +80% attack (w.r.t his base attack). It starts from 0 and can go up to 100 on the Agitation counter (The little fury counter on him). However, the only way to increase this by hitting an opponent having buffs with a Medium Attack or by getting hit. Each of these actions increase his Agitation by 5 per action and it falls off similar to Gladiator Hulk (it lasts a bit longer on Villain Champions, though).
His other important aspect is his Sig. Ability, which provides him with either a Fury buff, increasing his attack by 30% for 10 seconds (at max. sig) (If Man-Thing is above 50% of his health) or a regeneration buff, giving him around 4.5% of his health (at max. sig) (if Man-Thing is below 50%) over 10 seconds while launching a special attack. Keep in mind that these don't stack.
Also, I would recommend using Mystic Dispersion (which is a must, IMO while playing him) at 3 or 4 and 2/3 or 3/3 in Despair Mastery for maximum benefit. Especially Mystic Dispersion (MD), since for buff heavy match-ups, Man-Things converts opponent's buff to Toxic Pustules and then it can be nullified, which means he can gain a lot of power really quickly.
The ideal rotation with Man-Thing (provided the opponent doesn't generate any buffs (by his abilities or by nodes) seems to be -
MLM until Special 1 - Launch Special 1 - MLM - MLH - Launch Special 1 rinse and repeat (again, have at least 3 points in MD)
Also, note that you chain Heavy Attacks like Wasp from a basic Combo (However, this has a 5 second cooldown) this due to his 1 second Unstoppable buff he triggers while charging it.
Also for longer fights, throwing a Special 3 in between can bump up the damage up by few notches.
He has a nice set of utility (but not truck loads like Black Widow (Claire Voyant) , which includes -
1. Buff Manipulation - Man-Thing's best ability. If The Opponent has lot of buffs on them, Man-Thing becomes a very fun to play character. Man-Things's damage and power gain increases exponentially to the amount of buffs the opponent has. The best rotation then seems to be -
MM (Until Special 1) - Launch Special 1 - MH - Special 2 (if the opponent has lot of buffs)/Special 1 Spam (one might have have throw a few MM combos to get to a special attack threshold at times). (Again, Mystic Dispersion at 3 or 4 is a must).
See examples -
https://youtu.be/BeGR6O7gDVQ?t=652. Armor Breaks - Although the armor rating reduction is not that great (until you can ramp upto 11-12 armor break debuffs) but the duration it lasts for makes it useful against Iron Man (Infinity War), KIllmonger, Emma Frost match ups.
3. Heal Reversal - Poison Debuffs can mitigate healing, combine that with armor breaks he gets when Man-Thing stacks it up and you get good heal reversal with the Despair Mastery (Provided, the opponent is not Poison Immune).
4. Bleed Immune - Pretty obvious. Makes it safe to run for Biohazard and Caltrops nodes with him along with other benefits.
5. Armor Break Immunity
Also of note, he can ignore critical damage rating of champions when attacked and has 15% chance nullify buff on contact (this includes his Toxic Pustules) which makes Man-Thing a decent AW defender on certain nodes.
However, Man-Thing has a lot off weaknesses. Let's go through them and see whether it justifies people's disappointment -
1. Zero Armor Rating and a bit low Block Proficiency - This makes him easy to beat by any good high damage dealing who doesn't rely on critical hits. Champions like Star-Lord, Hyperion, Hela and so on. However, this makes him to take lots of chip damage while blocking. This is offset by his massive health pool, but it means that Parrying with Man-Thing is not a good option.
2. Small Regeneration when below 50% - Since the regeneration doesn't stack it makes it more of a deterent while fighting champions like Void, Captain America (IW) and many others. (Which is a good weakness from design perpective).
3. Low Crit Rating - This makes Man-Thing very bad with his raw damage. This is offset by his DOT efffects. However, therein lies the problem...
4. Poison Immunity - This reduces damage output of Man-Thing by 70-75%. This wouldn't be a huge issue if it wasn't for the fact almost one-third of the Cosmic Champions are poison immune and the main source of Man-Thing's damage output is Poison.
This leads me into the second part of the discussion - Poison. The first few champions to have Poison abilities were Abombination and Scarlet Witch (With her awakened ability). Which was followed by Dr.Voodoo and Howard The Duck on 2016 (Randomly inflicted on Special 2) and Archangel, King Groot and Green Goblin on 2017. While 2018 had no champs released who have Poison abilities in their tool kit. The next champion with poison abilities was released after 1.5 years from Green Goblin (which was Diablo). The champion after Green Goblin who had good poison abilities was released 2 years after (Mysterio). During the same time, lots of champions started having Poison Immunity (especially Cosmics and Tech).

(Also, special thanks to Ricardo Slugberg for designing AuntM.ai)

We see that out of the above released champions (from 2016 onwards), only Dr.Voodoo, Diablo and Man-Thing (now) have Poison abilities. Dr.Voodoo has tons of utility which bypasses his lack of damage in poison immune match-up. While Diablo is hated by everybody (We will come to him someday, if this series continues). Which now leads up to Man-Thing.
If we look up the data, around 20% of the champions in the game have some form of Poison Immunity or resistance. Out of which, 33% are Cosmic champions.
I think that Poison as such is not a good fit for the Mystic class unless it is an additional piece of utility and is better left with Tech and Science characters.
Does that mean that Man-Thing is a badly designed champion? Yes, in my opinion. The champion spotlight on Man-Thing states the design philosophy of Man-Thing as "is all about punishing Buffs, whether they came from its opponent’s kit, or whether it placed them on the opponent itself." - that implies that Man Thing is designed to be a Cosmic Killer. By this philosophy, Man Thing's poison not applying on one-third of cosmic champions is not a good sign and hinges towards a bad champion design. Now, I am not saying that Man-Thing should be that really awesome do-it all champion or he is not enjoyable to play. But what is supposed to be his best match-ups are gimped in damage.
So, does that mean making a Mystic Tank with DOT effects and utility is not possible? No, in fact this champion needs a can do what's intended of him without changing much.
Make his poisons into incinerates and have each incinerates have 20% reduced regeneration potency.
Also, it seemed like the champion designer struggled to make Man Things's Signature Ability work. So, here's my solution if II were in his shoes - Have the champion gain 25% less power while attacking into opponents as his passive (not included in his signature ability) and have his Signature Ability benefits stack. This makes more balanced and fine tuned. He becomes a great option for nodes like Resistor (not as great like Symbiote Supreme or Dr.Voodoo, though) and have bit more firepower for what is supposed to be a Damage Dealing Mystic Champion.
Also, his synergies are cool. Does't add anything for Man-Thing that's crazy or necessary to play, but are good benefits in certain Match-ups. His Ghost Rider synergy and King Groot synergy seems beneficial in most situations.
So, In conclusion - The main issue with Man-Thing is his Poisons due to many poison immune champions (especially cosmics). Otherwise, in other non-poison immune buff heavy match-ups (which is not a lot), he is enjoyable to play and not too overpowered. I would place him D tier (or Pretty Useful Tier, as per Seatin's Tier List). A bit more fine tuning can him viable in the current meta.