I'd suggest switching from the RoundStartPost timer you have set up to hooking the arena_round_start event instead, which fires right after players are able to start moving.
Reason being is twofold. One, one less timer to deal with. Two, tf_arena_preround_time is not always set to 10.0.
Also, not particularly important since developers can change it themselves, but you might want to put the section of code that causes damage dealt to minions to hurt the Boss into a case statement thing by default. I'm sure not everyone would want that as a global effect for all bosses. EDIT2: Actually, isn't the entire concept super broken if a Spy backstabs a minion?
EDIT: What's with the random line of code "blocking" the Persian Persuader in Handler.sp, line 1028?
Code:
if (GetIndexOfWeaponSlot(base.index, TFWeaponSlot_Melee) is 404) // block Persian Persuader