You would have to hit with the mf swap weapon last for the mf to count.
I would just use it to kill him completely. Thats what I do.
Or atleast get the last hit with the mf swap.
I am pretty sure you wrong
It counts your mf % in the moment monster die. It'd be a weird logic for the game if merc gets the last hit to calculate which hit before that was yours and with what weapon...
So if i cast a meteor and then i swap to alibaba the meteor will kill mephisto and it will count my mf with alibaba