If the creature with mentor leaves the battlefield with mentor on the stack, use its power as that creature last existed on the battlefield to determine whether the target creature has less power.

If your life total is brought to 0 or less at the same time that Truefire Captain is dealt damage, you lose the game before its triggered ability goes on the stack.

Truefire Captain’s last ability will trigger even if it is dealt lethal damage. For example, if it’s dealt damage by a 7/7 creature it’s blocking, its ability will trigger and Truefire Captain will deal 7 damage to the target player.

Mentor compares the power of the creature with mentor with that of the target creature at two different times: once as the triggered ability is put onto the stack, and once as the triggered ability resolves. If you wish to raise a creature’s power so its mentor ability can target a bigger creature, the last chance you have to do so is during the beginning of combat step.

If the target creature’s power is no longer less than the attacking creature’s power as the ability resolves, mentor doesn’t add a +1/+1 counter. For example, if two 3/3 creatures with mentor attack and both mentor triggers target the same 2/2 creature, the first to resolve puts a +1/+1 counter on it and the second does nothing.