Tank. If they can get your infantry with an excess of small arms fire, and lacking in the anti-tank, mount up your commanders in a land raider. Perhaps use the drop pod for a dreadnought, or a regular terminator squad with storm bolters and an ass can, while you load the command squad into a land raider. Also, bring something to the table that Dorn would approve of. Vindicators and Whirlwinds help to even the odds against armies tailored for bolterspam. They've got two tactical squads, unloading two heavy bolters and nine bolters into your command squad? Pie plate from whirlwind, and they're pinned, pie plate from vindicator, and marines start dying.