I think camping in the tunnels near the docks (the one with the 2000 point door to Jug) with the Magmus Operandi (PaP'd Magma-Gat) and The Sweeper (PaP'd Blundergat) is the fastest way to reach higher rounds. I personally use Alchemical Antithesis and Cache Back to replenish my Magma-Gat ammo and use The Sweeper to deal with Brutus whenever he does spawn in (as it one shots him, regardless of round) I managed to get to round 62 before I decided to call it quits and stop playing, and I think there's potential in a strat like this. The only problem is the luck required to get 2 blundergats, you can obviously just get the free one with the easter egg, but you need to hit the box to acquire the second one. That's really the only hiccup though...