This is a excellent guide, I'd also like to add

Round 11 - Black Magnum (headshot a nexemis twice and they die)

Round 13 - M32 (great for mobs)

Round 15 - Golden Punisher (double barrel shotty)

Round 17 - NAPALM!!! (Nothing except for hacks give you a better combo kill but recharge takes too long)

Round 20 - Minigun (Get the real Mini gun just like spy hunt)