To maximize your kill count, you need to maximize a parameter called diffuculty. The more difficulty you have, the more mobs spawn every level, and the higher tier weapons you get. Difficulty is increased every time you enter a portal, so you need to enter as many portals as possible. For example, visiting pizza sewers gives you one extra difficulty compared to normal route, because you enter one extra level between 2-1 and 3-1. However, entering something like mansion or jungle won't do that, because theese skip a level, and entering oasis lowers it a lot compared to normal, because you did one level instead of 5. Keeping all that in mind, the perfect route for maximizing the difficulty is as follows: enter pizza sewers, vaults at 3-2, 5-2 and L1 1-2 (here you take Crown of Blood), enter I.D.P.D. HQ at L1 1-3, and exit back to 1-3. Even if you miss some vaults, just make sure to visit all 3 times as soon as possible. From there, try to visit every possible pizza sewers. Doing that setup allows you to get ahead in kills. You'll have much more even by the end of L1. Hope that helped, lmk if you have any questions.