If you've played Pokemon, it shouldn't be too hard to understand. You have to weaken the enemy substantially (and when I say that, I mean A LOT.) and just hope it works. You can increase the power of the attack using AP points to increase the effectiveness of it, and having the Rice Cooker capsule equipped on Standby to improve its success rate a bit. Once you get the Scouter, you can see the enemy's current HP and that makes sealing enemies much easier. I still fail occasionally, but I've been having better luck since I got it to Lv3. As for a reason to seal all the enemy types, I don't know that, but I'm currently trying. Also, as Tien gains levels, the damage won't be so bad. Out of 1120 HP, Tien loses about 200 HP per use.