This is not as tough as it may sound; this can be done in one 3-ball game (with possible extra balls) instead on 1 ball. The number of berserkers being hit as shown in the status report will be reset on ball loss, but not in the progress of the achievement
This is done during "impare The Enemy" where vikings appear on the playfield.
Shoot 20 of these berserkers to complete the game-mode.
Relight the IMPAIR letters by shooting the right mini loop to restart Impair The Enemy and try to shoot another 20 of these guys, rinse and repeat until the achievement pops.