Click HERE to find a partner to boost online.
If boosting, you'll want one of the players to win until they rank up to level 2. Next, have the other player win until they rank up to level 3. Now have the first player win again, and both of you should have all 3 of the online achievements. The easiest way to do this is to send out your hero to destroy the enemy gate, and then have a fast troop pick up the enemy flag.
If you doing this achievement legitimately, you'll want to search ranked matches until you come across someone who is ranked a higher level than you. If they are not, you should be able to back out before the match starts. Once you enter the match, you'll need to win the game to get the achievement, which can be a little difficult as you won't have any upgrades online. Use a good castle, and try to plan a strategy that will work for you.