I just discovered that when using Bwonsamdi’s second skill (Irrefusable Deal) to become immune and then return all mercs to the bench at the end of the turn, it means that the AI can only return 3 mercs to the board next turn, even when it’s an elite encounter where the AI starts with 4 mercs on board.
That substantially reduces the difficulty for elite encounters, especially on Heroic and/or at role disadvantage. Particularly if the AI decides to target the now immune Bwonsandi, which it often will since Bwonsandi has low stats!
Should work well for boss fights with more than 3 mercs on board as well (although it won’t help against boss minions) - will try that later!