Power Weapons, Plasma Pistols, Haywire Grenades, and Brightlances. These are the elements of Space Marine destruction. Rob them of their 3+ save and their 14 Armour all-around vehicles and you've just won the day. Shoot up small squads in Venoms with Leaders and Special Weapons... but make sure you aren't in the open, Bolters can and will shoot them down. Take Death Jesters in the back with Brightlances to take out armour and transports, or even a dreaded Harlequin Wraithlord, that takes on average 9 Lascannon hits to bring down! Don't take Masks, Hallucinogen, Tanglefoot, or even Plasma Grenades, nor Harlequins' Kisses, and take only the bare essentials listed above to maximize your numbers.
Do these things and at the rearguard of your armies will be a heap of power-armoured corpses.