Killing

Hit them so they end up with 0 health.  Below 21, they are in the hospital and you can't hit them there.  So it has to be a strong hit to take them from health 21 down to 0 in one hit.

how many tanks would it take to take -21

Depends on the opponent's strength.

Ambushing damage points vary from 1 to 5 regardless of the level of the offender.
 
Though this practice does transform one's personality into another type ...