You can't keep everything alive, at some point everything will be exposed if you want to use them aggressively (= anything besides hiding for the entire game and showing up only to march block and then hiding again), even Terradons. Two ways around taking damage:
1) Single Big unit
+ Takes longer to kill 25% for Panic tests, 50% for VP, makes it easier when charging, tons of rocks dropping at the same time can actually kill smthg more than only small units/warmachines
- Harder to hide/maneouver the bigger the unit gets, all javelin shots/rocks go on the same target
2) Multiple Small Units
+ If one unit dies, you've got another at least to finish the job, forcing the opponent to focus more magic/shooting on them, many more tactical options, more march-blocking, can target/shoot/drop rocks on many different units (= with 2 units of 3 Terradons, you could theoretically charge two different warmachines while dropping rocks on another two units that you fly above. Yes you can drop rocks while you charge as long as you fly over something), take up less space (= you can hide easier, or place your unit between two enemy units even if there's little space)
- Easy to make a unit panic, easy to wipe out an entire unit, harder to do anything worth mentioning in close combat, very easy to lose the US5 bonuses
I should mention here that I've barely played Lizardmen as I just started, and I've never used Terradons, but from all the simulations I've done in my head, comparing them to Harpies (DE player) these are the general guidelines I've come to think of and of course, I could be wrong
