Mentor trigger ordering
You can only get one Mentor trigger, the one from Blade Instructor.

If a triggered ability that targets something does not have a legal target when it triggers, it will be removed from the stack as part of the triggering process, and you won't get a chance to resolve anything beforehand. This applies to Boros Challenger's ability as there is no legal target when it triggers.


  603.3d The remainder of the process for putting a triggered ability on the stack is identical to the process for casting a spell listed in rules 601.2c–d. If a choice is required when the triggered ability goes on the stack but no legal choices can be made for it, or if a rule or a continuous effect otherwise makes the ability illegal, the ability is simply removed from the stack.