Actually a 5* King Groot is worthy! I wouldn't recommend to rank up a 3* KG or 4* KG bcos it'll take forever to kill the opponent. But a 5* KG is certainly very worth! U can also check out Richtheman why he also recommend KG highly (only 5* version onwards).
On the other hand, a 5* Psylocke is certainly decent for her power control. However, personally I don't like her that much (I only have a 4* version at R3/30). For power control, personally I still like my 4* dupped Magik with MD, and my 4* dupped Vision AoU the best, followed by my 4* dupped Dormammu, I seldom use HE but I think I would also prefer him slightly better than Psylocke, but this is just my own preference.
I'll speak on this. I pulled Psylocke right when she was added to the 5* basic pool, ranked her up, awakened her and play her in every game mode. She's great if you play with skill. She's the only champ in the game that can bring your opponent to zero power and keep it there. Of course, you can't get hit but that goes without saying. Is she the best option for power control? No. Are there better Mutant Characters? Yes. But Psylocke is truly underrated and should be considered for the total value she brings to the table. I like the way she plays, she has a very easy control mechanic and once I locked the opponents power, it's game over for them. Just basically use her as you would star-lord, don't get hit and you will be critting like crazy. Also check out this link: https://www.reddit.com/r/ContestOfChampions/comments/5zsm5q/initial_thoughts_from_a_5_psylocke/
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On the other hand, a 5* Psylocke is certainly decent for her power control. However, personally I don't like her that much (I only have a 4* version at R3/30). For power control, personally I still like my 4* dupped Magik with MD, and my 4* dupped Vision AoU the best, followed by my 4* dupped Dormammu, I seldom use HE but I think I would also prefer him slightly better than Psylocke, but this is just my own preference.
1) Her SP1 power drains and if in doing that the power meter goes to zero, your opponent is power locked (until you get hit)
2) Once power locked, every 5 hits of a combo gets psylocke a charge. These charges can be built infinitely, given that you don't get hit.
3) Build about 15-20 charges and unleash her heavy attack, see massive damage.
Her build up is slow but with power lock you can keep doing combos without worry. Her heavy atk has a long range so it will usually connect.
I have a 4* rank 4 psylocke but she is not my first choice since I have other champs. I still take her for power lock for tough bosses, worth a try.