I think the secret to getting a high score is just getting the right power ups and learning to keep monsters away so they can't attack you (power striker helps a lot with this). The power ups in order of importance (roughly) are bonus finder, telekinesis, score master, panic, power striker, uranium shells, bloodlust, ninja, reloader, extra health and field runner.
The rest of the power ups are pretty much useless in my opinion except for when they are absolutely needed to stay alive such as bandage and blood sacrifice. Never use immortality no matter what! Sniper, new weapon, and crossbow master are all useless, never get them unless there really isn't anything else to get.
The way I usually play is to walk around shooting people in the beginning with your handgun and eventually when it gets hard just basically stand directly above the fountain and kill all the monsters you can while trying to keep them away from you. When health falls run and get it and just keep doing that and you should be able to get over 1,000,000 after a few tries. You really don't need to move too much if you can keep the monsters away, I only really move to pick up items. I also try to save my rockets until a little later since the machine gun works really well when you usually first get it.