well you make it a challenge by insisting on a single pet only, so I’m assuming you mean this is as some sort of meme run. if you want a more solid hybrid pet build you want to use a few pets, I posted this one recently and can go up to Sr75-80 on ultimate, this was also really played in hardcore SSF, not a theorycraft:
EDIT: also FYI i happen to have an unreleased build on my HD that uses hellhound only by insisting on it, it goes up to SR65