Legion Warboss

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 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.

If the token can't attack for any reason (such as becoming tapped), then it doesn't attack. If there's a cost associated with having it attack, its controller isn't forced to pay that cost, so it doesn't have to attack in that case either.