Overall strategy
Always start with your post powerful skill (skill number 3). Your opponent will usually start by casting a glyph anyway. Next he'll try to cast his most powerful attack, so make sure you interupt it, or block it (incase he used Glyph of Concentration.) Next use Glyph of Concentration yourself, and then follow up with the second skill on your skillbar. Then you can spam skill number one till any of the more powerful ones are recharged again.
Also note that some attacks can be dodged, like icespear and stone shards. Yes, in some battles it does help to move your piece from left to right a bit.
Also, you can CHEAT with polymock. Whenever you get to pick your next piece you are able to cast spells. You can there for cast Wards and quickly pick your next piece. The wards will be present for a few seconds in the next round. This can give you a small edge on your opponent... if a very unfair one.
Most people will not have trouble with their first opponent, so lets start off with the second.
Plurgg in Vlox's Falls
Plurgg will always use his pieces in the following order: Fire Imp, Ice Imp, Kappa.
Use these following pieces in this order: Gargoyle, Skale, Fire Imp.
Blarp in Gadd's Encampment.
Blarp will always use his pieces in the following order: Earth Elemental, Ice Elemental, and Fire Elemental.
Use these following pieces in this order: Kappa, Scale, Gargoyle.
Fonk in Gunnar's Hold
Fonk will always use his pieces in the following order: Kappa, Aloe, Wind Rider.
Use these following pieces in this order: Ice Imp, Kappa, Fire Imp.
Now you'll have the option of fighting 3 more opponents. Make sure you pick up all three the quests.
Dune Teardrinker
He'll pick the following pieces in this order: Charr Shaman, Charr Flamecaller, Titan.
The combo I used which works very well is: Earth Elemental, Ice Imp, Kappa. But make sure that during the third fight you keep an interrupt or block ready at all times so you can stop Savannah Heat from being cast. That thing is a killer.
Necromancer Volumandus
Next up is the necromancer. He'll use the following pieces: Skeleton, Wraith, Bonedragon.
Counter this with: Aloe Seed, Earth Elemental, Kappa.
Grulhammer Silverfist
Grulhammer is hardest of all. He'll use the following combo: Dredge, Dolyak, Stone Summit.
Against him I absolutely recommend using: Kappa, Earth Elemental, Ice Imp. The Stone Summit will cast two nasty aoe effects, try and interrupt both of them when ever possible. They are way more important to interrupt than any of his other skills.
Master Hoff
Now it's finally time to take on the Polymock master himself, but boy is he easy. He'll use the combo: Statue, Naga, Mantis.
Pick the following pieces and you'll beat him with one hand tied behind your back: Earth Elemental, Fire Elemental, Ice Elemental.
Please note that since Rob wrote this Hoff has been changed and is no longer a pushover. He seems to randomly pick three gold pieces, it may even be the same piece twice. You just have to keep playing him with Earth Elemental, Mirage Iboga and Kappa until you win.
Congratulations, hopefully you'll now have beaten all the polymock opponents.