Backstabs do massive damage in the normal game, only an uber or play dead watch can allow someone to survive a backstab. Another feature of the backstab is that it is often described as an automatic critical hit.
From TF2 Wiki.net:
Due to the nature of achieving a backstab, it is technically
impossible to score a non critical backstab.
The large robots in Mann vs Machine can survive multiple rapid backstabs, and I'm wondering if there is a way to up the damage even greater. The spy can gain automatic criticals after a kill in Mann mode, and also a spy may use a canteen, medic, mark of death or jarata to gain crits or mini-crits.
However, if a backstab is already a critical hit then is there any damage bonus gained by crit-boosting a true backstab?
(Excluding face stabs, of course, which do gain from crit boosting)
Best Answer
Backstabs are always crits, and crits do not stack in any way, so critboosts will not help deal extra damage to giant robots. To get more damage out of such backstabs you will need to upgrade your knife.