Before you can do this you'll need to learn ACCUMULATE and (if possible) the JP-UP skill. You also need at least one monk with the CHAKRA skill for this to work.

Walk across any green dot until you are attacked, then kill all but one enemy. Next, find five squares that form an + and are on the same height level. Have the monk stand in the center with all other characters in the adjacent squares. Use accumalte over and over and when your attacked just use chakra to gain back your health. Repeat as desired.