MArcomage

I don't know in which deck this card can be useful. Direct tower damage decks usually doesn't have mages and Mage tokens (maybe except Thor - but it is one rare card). If you are playing Mage deck - you want to increase your magic as soon as possible, and card which destroys your tokens cannot help you with that. Destruction deck? Is there any sense in Destruction/Mage decks? I don't know - didn't see that yet. And secondary effect - -8 wall for 5 gems - is too weak for this cost.
My idea:
If Mage tokens are set
Enemy tower: -(3+N)
N = #Mage tokens / 15
Else
Attack: 5

I agree that this card is pretty meh. I think the only reason to use it would be in a destruction deck to proc the destruction keyword. If I were to suggest a buff, I'd say just make the primary effect a lot stronger, so it actually becomes worth it to burn your mage tokens.

I use it in my destruction deck. And I do use Mage in that deck as well (for the gem bonus), so I do get both effects, but it's true, it's not worth to burn up your mage tokens for -8 tower. To be fair, it's a nice concept for a common card, but arguably it's not very useful or efficient. NG has a point there, I like the idea he proposes.

most people tend to oversee the use of a few -tower cards in every attack deck :P having 2 of these or pairing it with an invisible squad etc makes sure that as you're about to destroy the enemy it doesn't help to fortify with a wall if the tower is also weakened

I agree with your point. But Arcane bolt is probably most ineffective tower- card because of that condition. You should play Librarian, then Arcane bolt, then Librarian again, then Arcane bolt again...
And if I'd want to add in my deck a common card that attacks enemy wall - I'd take Battering Ram: -11 wall compared to -8 and 5 recruits cost compared to 5 gems.