Really...

Did you defeat all the bosses non-lethal? Or did you just pull out a machine gun and fire?
It's much funner to use the MK22 and the Mosin to defeat all the bosses. (Especially with The End... Holding him up, traq him till all his stamina is gone...)

I found the motorcycle parts (all seven of them) to be very annoying when I was aiming for all the frogs... Not too easy to snipe all of them during a moving bike ride.

Also, you can try to do a 0 Alert, 0 Kill challenge... and that would be immense fun.