I've had so many botched runs because of those lil fellas and I don't know how to take both of them out quickly

No idea what a "benjaman" is, as that is not something present in ULTRAKILL.

That said, if you're looking to kill the 2 guttertanks in the final arena of 7-4 (inside of the Earthmover) quickly, I can tell you a good method of doing that.

Ideally you start with placing saws around one group of enemies as you're falling (either side, doesn't matter which). You can use the knuckleblaster to punch the sentry into the lava, and then the shockwave to blast away any filth, using another shockwave if you have to. This should leave only the group of enemies on the side you didn't touch, and allow the gtanks to spawn.

Something very important is you want to take out the filth and sentries as fast as possible, while the saws are still healthy and the magnet will persist for a while longer. This will allow the saws to completely kill the gutter tank on that side. This means you can turn your attention to the other side.

On the other side, a common strategy is to perform a "nuke" (shooting the core eject with the malicious railcannon) to clear out all of those fodder enemies as well as get some good damage on the gtank. After that I would personally follow up with shooting a coin with the slab piercer charged shot, or switch between projectile boosting and shooting a rocket at them.

The other one should have died, and with this one dead by your hands, you can now safely exit the Earthmover interior to bounce to the exit, watching the Earthmover explode. Hope this helped!