Magic: The Gathering - Duels of the Planeswalkers

You can do this in any mode, on-line or off-line. It is very easy to do with a friend on player match over Xbox Live.

Your creature for this achievement is Sphinx of Magosi which can be found on the Thoughts of Wind deck or in the new Root of the Firemind deck. The later is your best choice as the card doesn't need to be unlocked.

You'll need to add 6 +1/+1 counters to it and then attack you opponent. If the damage goes through then the achievement unlocks.

The Relics of Doom deck also has a blue creature that can achieve this: Master of Etherium. He gets +1/+1 off every artifact that you have on the battlefield. Equip him with some equipment artifacts and he will get well beyond the 12 damage you need.

If you try this alone, play against the green deck and use the Deluge spell when you are ready to deal the damage. This spell will tap all non flying creatures so your creature won't be blocked. Credit goes to lord verh for this tip.

If you have already gotten the planes walker achievement before downloading the expansion packs then there is a far simpler way of getting this achievement without grinding to unlock the later blue deck cards.
You should have a maxxed out thoughts of wind (Aka the chilli night deck). you would have needed all the cards to get planes walker, don't worry about the cards added by expansions.
There is also a good chance that you have access to a high level teeth of the predator deck on a secondary account as you probably used it to go through the Co op campaign.
If you don't have either of these, don't worry too much, it'll be harder but still possible.
Play Co op campaign, the first match using these decks and with the blue deck on the account you wish to get the achievement on.
You'll be playing the very first match, the one against the red deck and green deck.

Neither of your opponents have anything to insta kill creatures, they might use giant growth, some direct damage spells and rage to assassinate creatures but once you've pumped up one of your creatures this won't be a problem.

Use your blue counters to get rid of goblin gliders and giant spiders, nothing else in their decks can block your mostly flying army.
Your green deck has two jobs, build up an army of creatures capable of blocking any rush that the opponents might try and to use you blanch wood armour cards on blue creatures. If you have been dealing with the goblin gliders and spiders then you can pick any blue creature to buff up and the opponent won't be able to block them. In the end I used a cloud sprite who ended the game at 29/29.
Just remember:
Kill goblin gliders and giant spiders
If an opponents monsters are getting too big because of armour you can always return them to their owners hand or steal them using blue spells.
Consider using counterbore to take any troublesome cards (blanchwood armour, goblin gliders, giant spiders, rage, giant size) off of the table for good.

You should find the opponents have no answer to this after the first five turns. Once you're set up you will have full control of the field.