Hi there.
I'm new to the game and was trying out Extraction using only 1 marine, just to learn the game mechanic with a friend. When generating tyranids, we noticed that termagaunts seemed very innefective against the space marine.
When doing the calculations, we realised that Termagaunts can't actually hurt the space marine, as their maximum damage output (including the 3 for penetration) is 18, which is the same as the damage a space marine can absorb with no harm (10 Power armour, 8 for unnatural toughness).
Is this correct, or are we doing something wrong?