Nicely done!
For Minacell, yes, if he gets a turn with at least one Mindrette dead, he will revive them all. Note that if he only has to revive one, he gets his next turn much quicker than if he had to revive all three (with two being somewhere in the middle). If he gets a turn with all three alive, bam, you're dead. Just to save you the time trying - Deathproof won't help here, Total Annihilation ignores immunity.
As such, a useful strategy is to go all out on keeping one dead, while slowly whittling down the other two. Once you get to the point where you can take all three out quickly, do so and use that as your chance to pile on damage.
Also, Minacell has the Transient-like condition of being immune to physical damage unless hit with Weaken (I don't recall if he has the same for magical damage / Meltdown, I think he didn't). However, he doesn't ever "reset his shields", so you only have to inflict it once.
You're now in the final dungeon, so bosses from here on will all be very nasty (I guess there's one that's not too bad, but apart from that). ;) On the flipside, though, you've only got 8 or 9 storyline bosses to go (depending on if you've beaten a certain other one yet - Minacell and one other both need to be defeated before progressing, but you can do them in either order).