I was thinking of two ways to fix that.

The first and, less creative was would be to just remove its ability to attack/move attack entirely. I'm not sure how this would be since the unit could never attack a champion but still allows free steals on minions.

The seconds would be making the enchant last 0.5 turns longer. The turn enchant is supposed to be so that it takes a guy and goes invincible on the opponents turn, then your turn comes around and it is still enchanted so you can't attack and therefore have no choice but to move away. I never saw that you could just leave it there and it would unenchant on your turn where you just take another guy and eventually ravage through the enemy line since it is fundamentally unkillable (this may require a 1.5 or 2 turn enchant and the 1 turn is just a wrong number). Adding 0.5 to the time would make the piece vulnerable on the opponents turn. This would make it actually able to be killed and serve the purpose of "rush in, kill, rush out" since it would need to leave in order to not be killed. This would also make it more strategic since you would need to convince the opponent that something else is in danger for them to not take their turn killing it.

What kind of functional downside did you have in mind, something like the reaper or the elementals?