From your roster Omega red best for korg fight imo, but i used mostly Prof X in Tier 7 korg which is much easier cause of life transfer node, even Wolverine, Cable, both Mags and Apoc does well for me as i use mutants on Tier 7. So yes tier 7 is much easier. You can also use Guardian, as his attacks are energy based.
Your roster could handle Tier 8 at least, probably 9 and maybe 10 (although I have not seen it yet). But out of Tier 6 and 7, Tier 7 is easier.
Tier 10 is only a stacked Mangog with unstoppable nodes (The one where unstoppable gets given on a timer depending on distance between you and him, plus a few more I can't remember) Tier 10 is only one fight
Can do 9 with this roster, mine is smaller and can do the lot with one champ. 6-9 gets easier each time imo, the pi increases but the champs/nodes get progressively simpler.
You can always use white mags prefight on korg so parries stick and with life transfer you can mess up the sp1 dex and claw the fight back so it’s also a good fight to practice korg stuff.
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These are my rosters.
I find the Odin boss tiers faster to run through though
Coupled with his rock shield your damage is highly reduced, therefore reducing your life transfer healing
Tier 6 Korg is probably harder and actually has no nodes on him to affect the fight.
But out of Tier 6 and 7, Tier 7 is easier.
Tier 10 is only one fight
You can always use white mags prefight on korg so parries stick and with life transfer you can mess up the sp1 dex and claw the fight back so it’s also a good fight to practice korg stuff.