This seems difficult, but if you win every duel you play it will take a minimum of 125 duels to accumulate 1 million damage. Do to the insane AI you will probably get this way before "Lonely Duelist" or "Friendly Duelist"; you will rack up damage from duels regardless if you win or not and duels from multiplayer also count. You will get this with ease if you are going for all the other achievements.
Boosting method: If you want to go out of your way to boost this; I have a strategy for it.
Cards you will need: Neo-spacian Hummingbird, Mistbody
Summon Hummingbird and equip mistbody to it. On each turn activate hummingbird's ability to recover lifepoints. On you friend's turn have him attack hummingbird. Since Mistbody is active, he won't die and your friend deals damage to you, counting towards his 1 million damage. Repeat this until it unlocks or your decks run out.
Having more than one of each in your deck will make them easier to draw and make this easier to get started. Only recommended if you boost all your win achievements.