Harsh Mentor
Oracle Text
Whenever an opponent activates an ability of an artifact, creature, or land on the battlefield, if it isn't a mana ability, Harsh Mentor deals 2 damage to that player.
Card Rulings
4/18/2017 Activated abilities contain a colon. They’re generally written “[Cost]: [Effect].” Some keyword abilities (such as equip) are activated abilities and will have colons in their reminder text. An activated mana ability is one that produces mana as it resolves, not one that costs mana to activate. Notably, exerting a creature isn’t an activated ability.
4/18/2017 Harsh Mentor’s ability doesn’t trigger when an opponent activates an ability of a card in hand (such as cycling) or a card in a graveyard (such as embalm), even if that causes a card to be put onto the battlefield.
4/18/2017 Harsh Mentor’s ability resolves before the ability that caused it to trigger.
Maybe a sideboard card against some fast combo's but I doubt it.
Yeah, have fun with that...
The only problem I see is that burn is already pretty crowded for what creatures to play. This goes up against some tough competition with Monastery Swiftspear, Goblin Guide, and Eidolon of the Great Revel. There are already other powerful red creatures that aren't making the cut anymore, like Young Pyromancer.