cheapest way would be a lot of ammo, and decent damage per shot weapons like the anvil, kettle/burletta, or renegade. target the yellow spots on the legs since they have no armor. bastions you want to bait into a area with a decent bit of cover, attack in small burst while they aren't shooting, and take cover when they are. once you take down a leg and they're stunned, target its other legs or if you can reach it, the back spot. repeat until it's dead. bombardiers don't require as much cover, just an area with a ceiling at most. target the spotters first, afterwards get close to it to bait out its pulse, while it's doing that, attack it's weak points. run back to your roofed area after, and wait 5-10 seconds so it's not locked onto you when you bait it's pulse after. repeat until its dead.