Undergrowth Champion is a 2/2 creature with an ability that reads:
If damage would be dealt to Undergrowth Champion while it has a +1/+1 counter on it, prevent that damage and remove a +1/+1 counter from Undergrowth Champion.
If Undergrowth Champion had 1 counter on it (making it a 3/3) and was in combat with another 3/3, how much damage would Undergrowth Champion deal? Would the counter being removed cause Undergrowth Champion to deal 2 damage? Or would the removal of the counter take place after damage amounts have been assigned? Would the opposing 3/3 live or die?