After an hour of trying, I cannot kill Slezz with the GOG version. She *always* closes her arms to block the bombers regardless of when I turn the water switch. 3 walkthroughs are no help, just give vague instructions (flip switch) that don't work and say not hard. If so, what is the real secret?

I don't recall seeing her do that, myself.
When she blocks the bombers, do they explode on her arms, or do they just sort of hit them and then fall? If the latter, maybe you could try grabbing them again while they're in midair and throw them at her again? That would be my suggestion.

Do you have diluted vision on? I found that it messes up your timing, causing her to block even after you pulled the switch.

The trick is that you have to watch for the bombers, and quickly pull the switch before she can throw them. After she throws them, you grab the bombers with your harpoon and fling them at her.