You need to be able to survive Megid 3 times without healing 18 out of 70 times, assuming you have all the best equipment and Chaz is at level 99 with as many Sol-Dew as possible in your inventory.
But, you wouldn't know which turns you'd be able to survive them, so you'd actually need to do it 18 times in a row (since if you guessed wrong once, you'd die, so this removes the guesswork aspect of it). The probability of Megid doing 211 damage or less is around 1 in 10, so...
Your chances of winning that battle are around 1 in one septendecillion or 1 in 10⁵⁴.