When starting ult as a fresh lvl 80, unless you somehow found 125 hp mats before lvl 80, you're gonna have a pretty hard time surviving anything in the later stages like ruins or seabed. Ep4 is going to be annoying too because of those sudden dorphon charges.
In ep1 the monsters are generally more straightforward, and with a strong party you shouldn't have too much of a problem staying alive for exp. You can probs get a truckload of exp from TTFs or popular (maybe? dunno if it still is) Froozer hunt in Tyrell's Ego Forest part.
In ep2 the miniboss monsters will pound you hard and most likely 1hko you until you have ~800 hp, and in tower, pretty much anything will 1hko you. If you found a nice hit hell weapon, you can still be helpful but most of the time, the rangers will hell needle through most of the critters so you might not get the exp you wanted.
In ep4 I think for crater, you really only need 720+ hp to survive dorphon charges, and in sub desert, you just need to watch out getting frozen and pummeled.
When you're low level, imo the best way to help is using a demon raygun or bringers rifle because you usually wouldn't have enough ATP to hurt Ult monsters fast enough. Once you reach ~140+ it starts to become a lot easier. Your mat/mag plan also helps a lot in how fast you can actually reach that nice ATP range for ulti.