The sgt isn't invincible before you initiate the wave, thus if you kill him, you cannot trigger the wave, nor can you reset the dungeon.
This only occurs after you do the dungeon once. He is initially invincible, but after you fail/complete it once. He's vulnerable to a TK.
This only occurs after you do the dungeon once. He is initially invincible, but after you fail/complete it once. He's vulnerable to a TK.