Unfortunately Skirmishers only get the -1 vs enemy shooting if their unit strength is 1. Ripperdactyls and Terradons are Monstrous cavalry so their US is 3 per model (Same thing with Bretonnian Pegasus Knights). Enemy Fire (Skirmishers) p.185 Rulebook "Due to their comparatively small size and ability to make use of all available cover, enemy units shooting at a unit of Skirmishers that consists entirely of models with a Unit Strength of 1 suffer a -1 To Hit modifier."
Under skirmisher rule is written that is applied only to US 1 units now. Skirmish is just a way to deploy and move/charge. Edit: sorry hadn't updated och hadn't seen the new posts =)
Yeah I think lore familiar is required. That 800 points is very difficult. Options: - drop lore familiar - drop armour of meteoric iron - drop skink hero (RRR on Slann) and no terradons - drop RRR. None are particularly good, I personally would say that's a "worst to best" but gees that's rough. Shame our Slann is so overcosted.