So the "with lesser power" clause on Mentor is a restriction of the targeting. That means it needs to be true when you try to target as well as when it tries to resolve. It may also be relevant to note that all the triggers go on the stack at the same time, even though they resolve individually.
Assuming there is nothing else relevant in play, you will have Aurelia as a 4/5; two 2-power creatures; and a 1/1 goblin token at the beginning of combat. When you attack with all, you trigger Mentor on all three creatures that have it. Aurelia has three legal targets, but the Warboss and Challenger only have one each: the token. This means you can't have the Challenger target the Warboss or vice versa, even if you planned to put the counter on the one doing the targeting. If you had the mana to pump the Challenger first, then you could do it how you wanted, but not without other interactions.
HOWEVER. What you can do is stack your triggers thusly:
This means that Aurelia will make the Warboss into a 3/3, then the Challenger will make the Goblin a 2/2. Then the Warboss will still have higher power than the now-larger Goblin, letting you pump it up to a 3/3 as well.