An update: I don’t remember if they fixed it or not, but the ‘cheese’ does not work against most bosses… apart from C’Thun (on any difficulty). Let me explain further.
As far as I understand, there’s an allotted number of mercs each side is supposed to normally start with or have in play, unless something else goes on. The default is 3, however, it’s not always the case for the player (Nefarian and Heroic Chess event… Maestra also, I believe… am I missing something?), neither is it for boss fights (but it still is for Elite ‘trash’ fights, though). If mercs are sent to the bench, enough of them should be played to fill the allotted places (it’s not bug-free in case there are also minions, but that’s another story). If you lose some, you play some from the bench (or ‘hand’) before the next turn until you have enough in play or run out of mercs.
Most bosses, I think, have enough space to replay all their ‘adds’ immediately… However, C’Thun, as far as I understand, has only one place, despite 3 enemy mercs being on the board at the start, — so that he normally emerges not as a ‘reinforcement’, but only when all the others are gone. Thus, if you send them all to the bench by Bwonsamdi, they will come out one by one, making it easier for you. I don’t know if this is a bug or design flaw, but something to consider.