My definition for exploit is pretty simple: Would such a tactic work in a more reality based thinking?

Barricading a door, so enemies have a hard time getting through: Yes, surely -> legit
Trapping someone between 4 knee high coffins?: Hardly -> Game mechanic exploit

Stealing stuff from a trader and selling it back to him: hardly -> exploit

Creating a huge see of fire, so they will lose armor passing through?: Sure thing
Creating a huge sea of oil because oil penetrates armor making them unable to do shit: Hardly, because they just should be able to set the oil on fire.