Re: getting hit in infinite overdrive mode:
I tried going through direct assault using nothing but overdrive, and I found the problem. When you use infinite overdrive, you tend to get over 98% shot down. When you shoot down over 98%, according to the Gamefaqs cheats page for the PS2 version of Rez, you face a subtly harder version of the boss. I noticed that on this version of the second boss, when it splays out into a giant fan shape, and pinches up a frond or two at a time to shoot missiles at you, the harder version will use more fronds at a time. Overdrive seeks closest first, so it gets distracted by the missiles without ever finding time to take down the missile launchers, and that gives the boss time to open more and more launchers, which all suddenly have the opportunity to zero in on you. Moral of the story: no overdrive on that element of that boss. Overdrive is usually bad when you have a single spot you need to focus on amid a flurry. Another terrible place to use it is the big cylinder in Area 5, which won't go down at all if you stick to overdrive, and the flying worms also in Area 5, some of which will escape unless you aim for their heads.
Re: score attack strategy: No, the same strategy from Play mode will not do well for you. It's much better to always use 8x target locks than to shoot down every enemy. An 8x lock equals an 8x multiplier applied to every enemy in the lock. Do the math on that, it's huge.