Unless you have really large terrain pieces, cover will not protect Raiders apart from on the first turn. Most cover will only give Obscured Target to the Raiders, and this is of little use after the first turn as the Raiders will be moving fast.
For a Wych Cult I would say that using WWPs is the most effective way to fight with them. You can't do an effective Raider Rush because you can't get access to lots of Raider Squads, if you hurl 5 Raiders of Wyches at the enemy then there is a good chance that 3 will be shot down before they can deliver their passengers (and having all of them shot down is not unlikely enough to discount as a definate risk).
As a 2000pt list, I'd try something like:
(Starting on the board)
Wych Archite - Agonizer, Splinter Pistol, Combat Drugs, Shadow Field, Xenospasm, Haywire Grenades, Plasma Grenades, Gruesome Talisman
Warrior Squad (17) - 2x Splinter Cannon, 2x Blaster, Sybarite with Webway Portal
Warrior Squad (17) - 2x Splinter Cannon, 2x Blaster, Sybarite with Webway Portal
Warrior Squad (10) - 2x Dark Lance
Warp Beasts (5 + Beastmaster)
Warp Beasts (5 + Beastmaster)
Warp Beasts (5 + Beastmaster)
Starting in the Webway
Wych Squad (10) - 2x Blaster, Wych Weapons, Succubus with Agonizer, Raider with Horrorfex and Trophy Racks
Wych Squad (10) - 2x Blaster, Wych Weapons, Succubus with Agonizer, Raider with Horrorfex and Trophy Racks
Wych Squad (10) - 2x Blaster, Wych Weapons, Succubus with Agonizer, Raider with Horrorfex
Wych Squad (10) - 2x Blaster, Wych Weapons, Succubus with Agonizer, Raider with Horrorfex
Ravager - 3x Disintegrator
Uses the usual tactic - the Warriors FOF forwards in the first turn, protected by the 'escorts' (in this case, the Warp Beasts and the Archite). Portals are opened next turn, on turn 3 Wyches emerge to wreak havoc.
Lots of Warp Beasts here. Warp Beasts are cheap and useful, they are basically 'throw-away' units that are designed (in this list, at any rate) purely to draw fire and do some damage where possible - if they get destroyed, they are so cheap that it doesn't really matter. Their speed and large charge range makes them good for keeping Infiltrators and fast enemy squads away from the Warriors (same goes for the foot Archite, she/he can likewise FOF and get 12" charge with Combat Drugs. 2nd turn assaults against the main body of the enemy army are entirely possible too.).
Also, the Horrorfexes and Trophy Racks are worthless against Fearless armies, so ditch 'em for something more useful if up against their likes. You could go for Slave Snares in their stead, or just use the points to buy some more Warriors or kit for the Archite, etc.