Of course he can’t charge its Bloodthirst, Hellfox is not a pure sweeper! Hellfox requires strategizing around: pair her with a Stun Bomb monster, so that once the monster sacrifices himself, you can get one kill with Double Retribution.
Then, use Rebirth on a teammate (possibly a teammate with a strong Revenge/Entrance effect) and Hellfox is already charged and ready to rumble!
This works even better if you have a Give Turn monster.
If I remember correctly when I was around your hero rank, I had only 2 leg and that was valza and I never fully evolved coz I didn’t know stun converter was a thing and tago ,maximize the SE’s you have, use strategic team building and not just aoe spam coz all I did was aoe spam, you have that megabomb SE pair it with an one-on-one yeti, those poison massacres SE aren’t pretty good I would just put them out or near that poison eater mons to support it, your team is also very weak to stun put in some absorbers besides that lion ,and get a galvbane
I didn’t get my first legend until after I beat the main story lol. This game isn’t stacked that hard against unfortunate players; there’s plenty of room for ingenuity.