There is an earlier thread, I believe, here on the Video Discussion forum that suggests that you use a step bench or high step for the down-low portion of the squat-thrust climber, and several others responded that that seemed to help.
That being said, IMHO the only way to do terminator climbers, especially with that fiendish jump that Cathe throws in, is to do them. If you're going to modify anything, simply cut down on the reps to a number you can handle, and as you get better at them add on one rep, then another the next workout, and so on.
I remember when Boot Camp first came out, that was one of the drills I couldn't finish as produced the first couple-three times I did it; however, I used those early failures as my cue that this was a drill worth working toward. I made sure to do that drill (actually, all the Boot Camp drills) at least once a week ever since that workout came out. And I also made sure, when Drill Max and later 4DS-Boot Camp came out, to do those drop-down drills as frequently as possible. That has had great benefits to my power capacity, and I'll bet a similar discipline can have rewarding results for you as well.
A-Jock