Cards target the front card from the opposing team, with a few caveats.
Snipe ability targets the first ranged or magic opponent.
Sneak ability targets the last opponent.
Melee cards can only attack from the front position, which is why frozen soldier was useless until spineback turtle died.
Ranged cards can't attack a card directly in front of themselves, so as we saw at the end there, peacebringer couldn't attack frozen soldier. He just stood there eating dirt until he died.