If you have two mobs, and you skillchain one of them, and say you MB a -ga nuke (or perhaps wiser, a Horde Lullaby or Sleepga) or something? It'll only MB on the thing that got skillchained, will land normally on the other mob. You can't MB on anything other than the target of the skillchain, so you can't MB cures to party members because you're certainly not skillchaining them.
You can, however, MB offensive cures used on undead enemies, on Transfixion/Fusion/Light skillchains. Pretty sure that it's ineffectual to MB a cure on anything not undead, be it a player (in ballista) or any other mob. You'll just get the cure-has-no-effect spiel.
But I'm not much of a ballista player so for all I know, curing your opponents in ballista (however illogical a concept) actually heals them.
Regarding the Water III getting "resisted" outright (as opposed to the normal concept of a nuke resist where it'll land but just do pathetic single-digit damage or something) ... that only happens against enemies *immune* to magic.
You probably saw it on either a floating eye, or a magic pot. Both of those mobs have an ability that gives them the status "Magic Shield", which temporary makes *every* spell (be it nuke or debuff, though I think Dispel/Finale can still go through to remove Blaze Spikes although not Magic Shield itself), be 100% guaranteed to get an outright resist.