With knockdown arrow in the game, unless the enemy has perseverance (usually only bosses have it), you can permanently keep an enemy down.

So yeah, you do win unless there are multiple enemies. Even if they are immune to KD, you can probably stun/freeze them with the elemental arrow anyway.

So if the level 18 you're talking about is a bridge troll, always using 2 electric arrow and one of the guy using fire attack to permanently burn him and remove his troll blood should serve as a good example of why having no armor = death.