I play DE, so i could help you out...
1) archon and incubi: stature/dredaxe, and d.str, wound the archon, and he insta-dies. incubi follow soon enough.
(this is asuming you can allocate attacs to certain models... i think ICs can in 4ed)
2) Wyches: not as scary as you'd think, just hope they don't get +1 str, rerolls to hit, or +1 attack in CCW. If they dont, just charge them with zerks or something, if they do tho... shoot the raider, then the wyches (6+ saves, HAHAHA now i feel bad...).
Actually, just shoot the raiders first chance u get.
With bolters.
3) Grotesques: use lascannons, Autocannons, and HtH on these suckers, cuz you can't hurt them with anything less than str 6. in shooting that is.
4)Stay away from darklances with tanks, and disintigrators. the Dissies are anti marine guns (only on raiders and ravagers tho, use bolters as suggested on raiders, but plasma on ravagers), and the Dlances are on troop squads, raiders, and ravagers(the former, use bolters, the median, use bolters, the latter, plasma)
Remember, he can only be glanced if he moves 6+" with his skimmers, but they go BOOM on a damage roll of 3,5,or 6. he will never move his ravagers that far, if he likes to shoot. rember, all his guns are crap, cept for the Lances and Dissies, and they shoot a max of 36," show him why range is good. use the anti tank wpns on ravagers and grotesques, and bolters on eveything else, and above all...
Dont let them take you alive.
Oh, and have fun, too.
-Drazhar