The chainsaw is the final melee weapon you'll find if you've been seeking them out through each mission. On Mission 8, you'll be tasked to destroy three plants from the start. The area you'll need to reach to find the item box containing this is outside to the very far left.
The ideal way to get to this area is to stay low, destroying one plant along the way (required to escape the room), and then climbing the platforms outside the small room where you fought an infected Tank in Mission 5. Make your way to a ladder on your right here, and then head left. You'll now be at the second plant with an infected Tank and some flying creatures. After this fight, head straight to your left and you'll find the box containing this.
If you aim for this specific plant last, you will only have ten seconds to get the weapon, as you're forced to a cutscene and cannot return to this area.
As for the 500 kills, you could work on this alongside Arsonist while also clearing up any missions you still need a golden glove for to obtain Perfectionist.