If you are expecting a strong possibility of a hydra, take a a Banner of Eternal Flame on some Cold One Cavalry or Temple Guard. At ten points it makes your elite Sauri (whichever type you choose) a good counter against the Hydra and if he doesn't bring a Hydra, you are only out ten points.
If you are REALLY worried about the Hydra give a Scar Veteran the Burning Blade of Chotec or take a magic lore with flaming attacks.
The Dark Elf list is too varied to make a lot of generalizations. Take the following with a grain of salt.
-They don't have high powered war machines so taking big dinos is relatively low risk compared to other armies.
-Their lack of war machines and their overall maneuverability means you probably don't want to overload your army with a large number of skink skirmishers (or Terradons and Chamo Skinks though they aren't useless because the Dark Elves have lots of elite units with low armor saves.
-While not as mighty as Slann, the Dark Elves can wield a magic phase that's especially potent, so know you are taking a risk if you go Slann-less. I'd recomend Light, Life or if you are feeling unorthodox: Heavens or Fire. I'm normallyl a big fan of Death and Shadow, but their high intiative and low toughness make Shadow a little weaker than normal and their high Iniative and high speed make Death a little weaker than normal.