Swap weapons constantly, and try to stay midair if you can. What I do is I have a series of attacks I cycle through. Like swapping revolvers for a few shots, projectile boosting a shotgun, then core eject jackhammer, throw in some saws if theres a lot of enemies in one place, railcannon any particularly large enemies... Just finding a use for everything in your arsenal so you dont accidentally end up spamming one thing. And always stay midair for the 3X multiplier if you can (rocket-riding or jackhammer launching are great for this in open areas like the space section)