Been playing a few more runs and wanted to expand my tips above a bit. Fire kills being double points means grenades, barrels that explode/roll, and the rocket launcher (L) weapon. Always get the rocket lawnchair when you get a choice between it and the machine gun.
Bulbs you shoot out kill the lights for the floor they are on, ironically shooting out a second bulb will turn the lights back on. There are breaker boxes on some walls and they kill the lights for every floor on screen, sometimes they are hard to see as they blend in to the background. Stage 4 has a couple at the last section that are easy to miss.
Double points in the dark applies to everything. Point items you pick up, kills, everything. When going to a floor plan it by hitting a light then getting the kill. If there’s a point item or two hit a light before grabbing them. Same for red doors, they equal 1000pts, in the dark = 2000pts.
You can milk points in most stages but you’re on a timer, it’ll appear on screen when your at a minute and countdown. Usually other than stage one you go through em fairly quick and might not realize. You can stand and melee the guys at the end of stage one, two, and four for a decent bit. You can do it stage five as well but it’s a bit riskier.
Also, use your jump to damage things. Don’t waste grenades on the debri on the floor that break and can give you items. I use the jump for the little robot guys quite a bit too.
