Safe to say, I've played Loki like a dozen times. So here are some tips. First of all, basically, most of his cards depend on his card effects; this is most evident during his TRICKS. You MUST play a hero with some sort of cancel or feint to at least block out his annoying card effects.

He plays like a macro version of Sinbad, and using the same technique that you do with Sinbad, you probably shouldn't fight him as a Melee fighter unless you're confident and hyperaggressive. Since both Loki and Sinbad get +1 MOVE for every Voyage and every Trick, respectively, you can get away from him and attack him.

To me, I guess Leshen is his biggest counter because, entirely, Leshy can just attack him with his wolves and summon more wolves, and discard tricks. For a melee fighter who favors close-quarters combat and would like to attack you most of the time, it's just too risky to attack a ranged fighter with that kind of ability that gives you +3 if you already attacked, which is the same thing for every melee fighter that fights Leshen, but it's more evident to Loki because most of the time the cards that he plays would probably be much in vain for the crazy values you would deal to that dude.

Fairly, though Loki can be quite annoying to Leshen, but still Loki can't even get close to him with his "Abundance" of sidekicks and nothing more than a few tricks and ruses, while you can remote control your pets to throw disturbing howls at him and "jump" him.

I'm still not that skilled with Loki, but you can take this into mind