I´ll tell you what´s the best way to obtain this achievement: Play with Goldmin. Goldmin is a wonderful character because his power: He can freeze completely the other player for a few seconds. That´s a really nice power, and a huge disadvantage to your enemy.
But there is one greater thing about him. Not only you freeze your opponent. If he was making a group of yosumins, he will stop doing it. Do you realize what that means? You can then go by yourself, and make the exact same group he was going to make, and then some more. This is the key for beating this mode easily.
You have to think that probably, you will screw up on some battle or another, taking 1:30-2:00 minutes. So try to beat as many opponents as you can at 00:30-00:45 seconds. It´s specially important on the first two-three battles, because it´s easier to just restart than to try to get better times on harder matches. You have to count that on every match, the CPU starts making a group. Sometimes is small, sometimes is bigger, but it can severely damage you from the start.
Your strategy will be to make as much big groups as you can, at least until filling you power bar. Once it´s full, you are nearly saved. Just keep playing and watch for you opponent´s cursor. If he´s going to make a great combo, use your power and make that combo yourself.
Try not to use the stop power if it´s not necessary, because it stops them from dying too, but the timer keeps counting. Only use the power when you see that the CPU will make a great combo, or that he´s going to make a group that will fill his power bar. You don´t want them to use powers, or you will lose a lot of time.
Playing like this, you should be able to make it in a few tries. Be really careful with the last combat. It´s Goldmin too and it can severely hit you with the same tactic. Against him, focus on getting the power from the start. You don´t want him to use his power, or you will have a big problem.