There are two ways you can cheat, if you get fed up.
The first is get a PC savepoint off the internet for the next level, as you are so near the end of this level, all you get after killing the two centaurs is a short cutscene.
The second cheating method if I can remember is.... to run out and dive off the edge of the pool swim quickly, down and down, dodging blasts from the centaurs and enter the underwater tunnel and throw the switch in the dry cave below, stay there for a while (how long ?) and one centaur gets killed by the other, so you are down to one nasty left.
This is where I forget how to get rid of the last one when you surface, other than fight it ... I have so forgotten this cheat, DOH !