So about DOOM You gain points used to spend on upgrading weapons via completing the challenges for each level, you can see these in the pause menu and are often Kill X via Y / While doing Y and so on. You also gain these points from completing combat encounters, if you collected 100% of them you would have more then enough to get every upgrade, this means you can miss a fair few and still cover all the upgrades you want. The Chainsaw is best thought of as a mechanic and not a weapon. It uses fuel as ammo and bigger enemies require more pips of fuel to attack. If you have enough you do an instant kill and the enemy will spill out loads of ammunition. Basically you can use it to take out a harder enemy instantly but its most used to regenerate ammunition the same way melee kills are used to drop health once you've taken damage.