Wait until late game to do this. MAG+999 takes one combine, and ATK+999 takes 4 combines (yes, 4, it sucks). You will likely get this while making enough MAG/ATK for your entire party. If not, go to one of the first battle points, you should be able to get them to 1 HP in one hit with merciful strike, and then immediately use capture on them. Put merciful strike/capture on as many of your characters as possible and go to town.
You can only have a max of 11 monsters at a time.