okay, i see i guess saurus are good then... they should survive against enemies like puny S-3 T-3 elves
This. The rules of 8th ed have been designed to make people take larger units. Even a unit of 20 I don't think cuts it for most troop types who used to go 18 or cut off at 20 maximum. You need to be able to absorb a few casualties and still be ok in the next turn. If you are building a list including a unit of say 20 saurus, and find yourself with 20 odd points left over, throw an extra saurus or two into the unit! Might as well, means a few more deaths before you lose a rank bonus.
it really depends on the rest of the army. for example if you have a life slann you can afford slammer saurus units, since the moment you lost the rank bonus you can cast regrowth where necessary, same with flesh to stone if a unit seems to be falling too fast. lore of light might save some saurus aswell if you get to I10 or ASF, but toher than these 2 lores you are going to need bigger units since once they die, they are dead. lore of shadows is arguably good aswell since it can reduce the S and WS of the enemy and add to the survivability of the saurus, buts its trickier and i wouldn't rely on it to validate smaller saurus units. i allways use 2 units of 20 saurus and 16TG+Slann with life (sometimes light for a change) and i'm quite happy with the results, i can afford to lost 5 saurus before losing attacks and flesh to stone/regrowth keeps the combat balanced. for lore of death/fire/metal/etc. i'd probably go for 25 strong units