Done this before and helped my partner do it. You need the following before you start:
-at least 1 friend
-timer
-your team size must be 2-3 people (no more than 3). This includes only you, your partner, and optionally 1 other. Preferably, you should do it with just you and your friend. This means it's best done in a 4-6 player game.
1. Send zombies until you get 1500 XP (you must constantly send zombies every 15 seconds).
2. Kill-steal mobs. If they're low on HP, kill them with your sword before a tower finishes them off.
3. Unlock the giant at 1500 XP.
4. Your friend should build expensive towers for you to sell. That's zeuses, leaches, turrets, necros, and the like. I'd say zeuses for their temporary use. If he can AND if the 1 other player is a random player, get him to build at spawn early in the game to ensure he gets more coins. If it's just you two, then you won't have to worry about this issue.
5. He should have those towers upgraded completely or as much as possible by now for you to sell. At 14 minute mark, your friend needs to leave the match.
6. Sell his towers and get the 6k coins you need. Send giant now.
Challenge complete. This took me several attempts (5-6 match attempts) to get. My partner beat it in 1 try.