It sounds like the defender was smart and put a lot of points into their big band’s HP stat. Ultimately it means you need a fighter with more attack to deal with something like that. Or, you need fighters who can survive while applying alternate methods of killing. futile resistance reflects a max of 6% max health as damage. That means it would take more than a couple grabs with xenomorph before the catalyst alone killed you. You must have gotten hit with something else, or just got really unlucky with the elemental shifting (which can happen, Xeno is a coin toss in effectiveness). If chip damage was killing her then give her more HP and defense or block proficiency to compensate.I did; i used my strongest atk char (a primed parasoul) and got killed by reflection. Bleed filia died by reflection before aplying much bleed. Xeno died to reflection (couple of grabs while trying to doom) and chip damage.
That catalyst is ridiculously broken!
Obviously, this won't work if you're dealing with Immunity node, but I have yet to face this challenge in the future...
Just a word of caution: high piercing against Frost Armor often results in major loss of life.I have found futile to be more of a damage per hit check as some others have already said.
lets go to the extreme:
Heavy Metal lvl 60 89k hp base
lets put him on the boss node and have +200% hp from moves which is already nigh impossible. this gives an effective ~540 k hp. so you need to do >6k damage per hit after defense and armor etc.
lets make it even harder and assume autoimmune and frost armor, so you cant use sketchy curse and armor procs very very often.
so this leaves you with hard hitting attackers only. but even in such an extreme case it is still quite easily possible.
1) Harlequin or Toad with special cooldown, piercing reaching 50% allows for infinite merry gorilla and battle butt. if those moves are lvl 12, you will definitely do more than 6k damage per move.
2) Summer salt or primed. Because they can crit for 50k or higher with their crit damage marquee. Couple that with ssalt free precision you will be fine.
3) Twisted mettle (my favorite counter). Use grudge and let the big bands hit you. their combos are long and you will easily get 3 armor or more which means zero damage received. plus if they are that tanky, their att and piercing stat is probably low. 4 crits received is enough for 4x 15s bleed which is enough to bring them below 50% hp and nullify the futile cata.
Harle, Toad, Twisted Mettle and Primed are quite accessible characters, so no complaints justified there on futile being impossible to beat. Tough definitely, but not impossible.