I used a second account, which only had access to the solid green deck (like mentioned already), and not maxed out. With my main account, I just have been trying to choose the best deck to counter the two decks I'm playing against. For example, when I'm versing the Black deck, I either go with the Blue (counter spells) or Artifact (prevents terror's and fear's from running the game). Playing against the Scales of fury deck, I would use the Elf (overpower with trample and numbers) or the Fire deck (haste and direct damage) to just blitz attack them. Using the green deck to augment my army, such as Blanchwood Armor and Giant Growth to help my creatures survive or rule. Best part about the co-op, you can share abilities (like putting blanchwood armor on the Flameblast Dragon, etc).
So far, up to the time I'm writing this, I am only missing 2 battles (expansion co-op) from obtaining this achievement, which I plan on finishing tonight.
Something to note, if you download the expansion pack, it is no longer "defeat Tezzeret and Tezzeret". With the expansion, they replace one Tezzeret with the guy Nicol Bolas, or something like that.. The new character they added to the game, whom you fight to finish the secondary single player campaign. But the achievement will still unlock once you defeat them, and when you check, the text description for the achievement will reflect the player change.
Edit: It is now the next day, and I was able to manage to beat the final two battles, but some advice for the final battle: I gave up using other decks and went with my solid white deck. Reason? The computer has to be cheating. They will get EXACTLY what they need to defeat you almost every single time. Both decks you are versing have 3+ Terror's (by my count) in them, which they LOVE to use in BULK to destroy EVERYTHING on the field, they also have Terminates, they both have a Platinum Angel, dragons, artifacts, and strong black and red spells make fighting these decks a true pain in the behind.
Artifact deck loves to boost the Glaze Fiend, the other deck just screws you up. They have a creature that takes your creatures when they die unless you pay 3 life to prevent it. Serious pain in the @$$
My game of goof.. They have 9 life, I have 4, they only had 1 blocker (5/5 Flameblast Dragon), I had 3 creatures, 2/2 bears, 2/2 civic elf, 11/11 regenerating troll (blanchwood armor, holy strength), all their creatures are tapped (deathtouch owl, glaze fiend, and something else i forget) except the dragon they just summoned. Accidentally placed second blanchwood on troll instead of other creature. Of course i lost next turn.
Winning Game.. no white creatures on field (got terrorized to hell again) Regen troll with blanchwood, kept killing things on attack, out comes Platinum Angel, they have just ground walking creatures and no flying (well, the ones they had i pacified) played my +3/+3 +Flying until end of turn on troll, made them negative life, then reached into my white hand and pulled out my...
Mass Calcify FTW
Quite easily the hardest match up in the entire game.