the slime can split with any Ra-magic attack if it's strong enough. it just doesn't do it all the time. it can split with grants even. but not megid, naturally
grants is good if you want to get rid of the slime fast because it can get rid of that extra HP. But to prevent splitting I suggest a powerful simple tech followed by grants. Or if you grants is fast enough just go for the double grants before the slime deforms into a pool again. So you might get another slime by splitting, more Exp and a chance to see a red slime.
ps: if you really want the slime gone hit it only with special attack from Hell weapons. this way it won't register a hit and deform until a good death attack connects. 95% death rate if special connects (sometimes you deal critical damage and no death trigger, weird...) But fast way to get rid of slimes.