The game has 16 levels, and you need to beat level 16 to win. However, levels don't progress 1 at a time, the level you are assigned is based on the combined level of all your soldiers. If a soldier survives a battle they will gain a level and be granted 1 skill point. So if all soldiers survive level 1 you will progress to level 4, if they all survive level 4 you will progress to level 7 etc.. So it's possible to do a perfect run and win the game by playing only 6 battles.

If a player doesn't survive they will be replaced with a new recruit. As you progress through the levels new recruits will start with more skill points so if you lose any soldiers or have a full wipe you won't be set back too far (and you can respec your soldiers).

If you abort a mission after any soldiers have died they will remain dead. However, because the game is only saved between missions you can "save scum" by resetting the cart and continuing the game with all soldiers alive.