Light of the Legion

If another white creature you control is dealt lethal damage at the same time as Light of the Legion, they're destroyed at the same time. That creature won't receive a counter from Light of the Legion's ability in time to save it.

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.

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.