Imho the easiest way to beat him is to stay in normal form all the time, possibly having shield on, unless:
1) You just bloated a merman, bound it, and need to push Mithala back until he starts sucking in. In that case, push him back a little by shooting at his head, change to normal, unbind the merman, and change back to energy, and shoot more.
If you are too slow for this, try to position the merman as far left as possible (without running into Mithala), and then switch to energy form and shoot at him until he starts sucking. I find this 2nd variant easier too.
2) After feeding him 3 mermans. If he is red and really, really angry, just keep shooting. I recommend charged shots, because charging and in the meantime evading the big, fast shots is a lot easier than spamming and trying to evade.
Actually, imho evading the big red shots is easier than those he uses before that -- if you swim slowly (just don't get hectic and jump around on the screen all the time), the shots will always miss you.
Ah, and don't forget about food - you should have at least 2x +2 foods, thats what i need when untrained. But i never use shield, its for wimps.
(I just beat him again, right now, to check this out and not miss anything
)
EDIT: One thing i forgot to mention: If you are in the pit, he grabs you after a few seconds; this gets especially annoying after feeding him 2 mermen and trying to get the 3rd one to bloat. Just push him away a bit if he's near the pit, that should give you enough time. And he also never moves when shooting. As only the last few shots are aimed down, you have lots of time (= about 3-4 secs) where he will neither try to grab you in mid-height, in the pit, stomp you, or aim shots directly at you.
