I just got it with my slime, but to use it, you have to use ALL of your mp..... that's kinda pointless and wasteful, even if it does 999 damage you won't have any more mp left

why all the hype about MegaMagic then?

I agree with you. Megamagic is an extremely overrated skill. Gigaslash is another one that is grossly overrated. Now quadhits that is an underrated skill that all monsters in your part should definitely have. Twin Hits is also good as it doubles the damage done. Big bang and hellblast are good too. Heal and healusall are a must for obvious reasons of course. Squall hit is also a good skill, though mainly just for metabble killing. Surge is a skill that is definitely good for one of your monsters to have too. I would strongly reccomend though people stay away from Megamagic and Gigaslash because like I said they are way overrated and IMO suck.

megamagic is only a good move if you are about to die in the traveler worlds , or if you know how much health a boss has left. mega magic should only be used freely if u have 10 elf waters or something crazy like that

Megamagic? Horrible as far as practicality, but it does do some great damage.

I'd only recommend it for use in boss fights as a last resort, but as for regular use...I definitely wouldn't use it.
Hellblast and BigBang are great. I also like Thordain, but I wouldn't consider that the best skill.