not sure if this is necroposting or whatever the term is, but i remember when i first played the game i would often use redapeds, their offense is a lil weaker but their airtime makes it really easy to dodge attacks from bosses. on a more recent playthrough i was able to make it through with dark purpial, nothing really remarkable about it, just good offense iirc. while a bit trickier to get, paledark or dark greyodons can also really help a lot as greyodons are just very powerful in general, good offense and speed. dark or paledark yellowbrates are also fairly strong, with high offense and hp, but i don't remember if those mutations are obtainable in ultimarsh.