One of the most annoying trophies in the game. You basically have two options of going about getting this.
1.The reliable one. First upgrade your damage carnival to max. Next, find a battle with a small enemy that won't die from attacks from your weakest characters. Eliminate all other enemies meanwhile building your blast gauge to 999. Aggro the crystal with Syrma or
. Assuming those two characters are too strong and will kill the last remaining enemy in one, go ahead and switch out to weaker characters. Wait until the enemy uses an attack that causes it to have a large wait time, guaranteeing you many turns. Use moves that have low wait times, like Clear, to blast off the enemy into the floating orbs. You may have to mix in some stronger moves to regain some blast gauge. While not foolproof, this is the only reliable strategy.
2.The random, but easier one. For this method I recommend fighting one of the large boss enemies in the Mugen Field rest stops. Save beforehand and proceed into battle, remembering to bring a couple weaker characters that won't kill the enemies in one hit. In the battle, build your blast off gauge to 999 again, making sure not to kill anything. Once you have it, position multiple characters in front of the boss and have another character go behind it and blast it off(ground level, don't aim up) towards your party members at full power. Majority of the time, this will not work and will merely net you a combo of around 50-100. However, every once in a blue moon the enemy will be trapped in a cycle of ramming all other enemies and characters around the stage a bit, giving you a high combo that will most likely be over 1000. I managed to get this strategy to work after about 2 hours of trying and ended with a combo of 1,080. If this doesn't work, simply retire and reload your game.
Hopefully with either method, it shouldn't take very much time at all, but you'll still need some patience. Good luck!