Fragger Level 21 involves a specific sequence of grenade throws to accomplish the level.
Step 1: First you must toss a grenade through the top entrance of the building so it rolls along the platform and nestles under the ledge holding the box of red tnt.
When the grenade explodes it should blow up the platform and the tnt should drop onto the other 2 boxes of tnt down below.
Step 2: Roll a grenade along the ground so it drops into the lowest tunnel of the building and nestles next to the tnt boxes. When it explodes it should wipe out the door that is blocking the four wooden boxes, directly below where the Fragger man is stood.
Step 3: Throw a grenade into the second tunnel up so it rolls along and stops near the four wooden boxes. This grenade should destroy the lowest box and therefore lower the fourth box up that was originally blocking you from blowing up the fragger man at the begining of the level.
Step 4: Throw a grenade into the 3rd tunnel up so it rolls along and blows up the hiding man.
Thats level 21 of Fragger complete on normal difficulty!