Cheapest priced copy (Guilds of Ravnica): $0.35 at Card Kingdom, near mint, their price list as of .
Boros Challenger 
Creature — Human Soldier · 2/3 · Uncommon · Guilds of Ravnica · #156
Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
{2}{R}{W}: This creature gets +1/+1 until end of turn.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Guilds of Ravnica | 156 | Uncommon | $0.35 | $0.49 |
| Guilds of Ravnica Promos | 156 | Uncommon | — | $0.49 |
| The List | GRN-156 | Uncommon | $0.35 | — |
3 rulings from Wizards of the Coast
- 5 October 2018If 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.
- 5 October 2018Mentor 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.
- 5 October 2018If 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.