A potion bomb, 10 of them or something.
He has to drop them near the guards to knock them out so he can make it out of the garden, however the effects dont last forever and you have a limitted supply (10).
I would use a step-based variable to determine how many steps the player has taken after dropping the potion and after 20 steps the guards wake up with a switch, the potion can be determined by having the player press a button within an X and Y range of the guard event, so you can drop one anywhere within an area, but when you step into the area the guard has a switch turn on and they start rushing towards you and when they touch you there is a hard battle or you just start again.
I'm gonna use this idea now :D