I posted this in 40K forums in the past, but thought it was worth the inclusion here:
Based on the 7th edition rules for artillery, when shooting at an artillery piece (or battery), all units with the artillery piece (including attached ICs and crewmen) use T7 from the artillery, along with their normal armor/invul saves.
Based on that, take the following:
-Thunderfire Cannon with Techmarine Gunner
-2 x Chapter Masters with AA, SMB, Storm Shield, and Thunderhammer
-Allied Draigo (LOW from a allied Nemesis Strike Force detachment)
Attach all the ICs to the TFC during deployment, then use Draigo’s Gates of Infinity to “bounce” them anywhere on the board (preferably somewhere that you have a Locator Beacon or is open enough to allow for a Scatter without a Mishap, say 12″ away from any enemy units or impassable terrain).
Now, you have a unit with 13 x T7 wounds protected by 2+ saves (12 of these wounds also have a 3++ save) and 2 x T7 wounds with a 3+ save against any enemy shooting…. a BS4 bolter on average would take 54 shots to cause a single unsaved wound, a BS4 Plasma gun would take on average 9 shots to cause a single unsaved wound, and a BS3 TLed Tau High-Yield Missile Pod (from Broadside Battlesuits) would take an average of 16 shots to cause a single unsaved wound, so this unit should be able to easily weather pretty much any shooting that comes at it.
On the same turn the unit “Gates” in it can release two S10 AP1 Orbital Bombardments (from the two Chapter Masters; use the Tactical Doctrine this turn to allow re-rolls of the scatter dice). Additionally, the Techmarine can shoot with his Servo-Harness weapons and Draigo can use his SB and, if you have the Warp Charges, use his Purge Soul ability to snipe a model.
The following turn, you can either split the ICs off and charge multiple different units (you could potentially charge three different enemy squads, tying up or destroying his entire backline in a single Assault Phase), or have all the ICs detach from the TFC (which can shoot this turn, since it didn’t move) and charge together as a single big “super friends blob,” benefitting from +2S (Hammerhand from Draigo… so that means S10 Thunderhammers, S9 AP2 Titansword).
Anyway you slice this, it is an extremely tough and dangerous unit appearing out of thin air in your opponent’s DZ turn 1 and likely cause panic and chaos for his battle plan by putting him on the horns of a dilemma… either he ignores this unit and gets charged/shot to pieces next turn, or he concentrates his fire on it and still does negligible damage, freeing the rest of your army to maneuver.