Northstar works great, but you don't need to rank him up
i used unleveled 7* R1, you don't need any damage, you just have to remove his charges and he will die.
if you genuinely like northstar as a champion, go for it, but for just one fight, there is no point, all you will get is more hp, since attack doesnt matter in that fight.
u can also use domino and quick silver, any dual /triple hit champion is faster for the fight.
notice now i said "faster" , because thats all it is, you can even do the fight with a 6* R1 Groot, no joke, Seatin did a video fighting Emperor Doom with Groot. so my advice is to not base your rank ups for 1 boss fight that can be done by Groot.
Silk
I used silk, she ate him alive. Honestly 9.3.6 doom was the easiest end boss we've had in awile.
Don't need to rank anyone. If you mess up you die no matter how much health you have and you don't deal damage. Just use your r1 northstar and revive spam with him
Thanks for info guys
Silk for the easiest completion. Her safety net is clutch
Rank 4 WWBN, makes it easy as well, double hit in medium attack and gains power just by holding it's position, when needing to use an sp
silk and Northstar works best.Bare in mind ranking up the champions dont matter that much as doom will one shot you most of the time and the only benefit from ranking for the fight is a small safety net so doom doesnt just clap you.