First of all I will share the node below so everybody can see the description.

As the node states, whenever healing is reduced or reversed by a debuff the potency of the regens increases and the attacker gets passive degen. Immortal Abom or ibom, on this node is an extremely niche fight. First of all you would need a poison immune because of his base kit. On top of that, ibom inflicts self poisons and since there is Lionheart, he degens you each time a self poison is applied on him. Furthermore, an awakened ibom starts off with 3 self poisons so you degen from the start and ibom has 300% more potent regen including willpower healing and buffet.
The only 2 reliable counters I can think of are Quake and Gulk. Maybe AA with good neuro RNG, Ghost with tech boosts+Hood and Warlock with very slow parry heavy gameplay and heavy boosts since the degen from lionheart is unavoidable. On top of that there is buffet to consider as well. Feel free to add more counters if you can think of more.
So my question is this: Why does such a niche fight exist? The poison debuff/Lionheart interaction should be reviewed to make this a fair fight otherwise it is too much to consider for a single fight. The degen is pretty much unavoidable and deals around 20% so you would die in around 50 hits just from the self inflict poison applying and causing unavoidable degen from Lionheart.
*** I feel the need to add this final disclaimer, yes the counters I wrote can do this fight with good gameplay but the requirements are far too steep to have only a few counters.