I've been tinkering for some time with a way to make a Seraphim Apocalypse formation, but also one that is in-line with an acceptable level. As much as I'd love for them to carry man-portable volcano cannons...yeah that probably won't fly.
Anyhoo:
Seraphim Descent Wing:
Formation:
3-5 Seraphim squads (squads must be upgraded to 10 models, all must be held in reserve and deep strike simultaneously)
Coordinated Descent:
After the first Seraphim squad is placed via deep strike, all other squads may deep strike within 12" of the first squad without scatter.
Righteous Rain:
As the Seraphim enter the battle, they unleash a devastating rain of fire from above.
On the turn the formation arrives from reserves, you may make a special Rain attack on any target within 24" of a Seraphim squad:
S4, AP5, Apocalyptic Barrage X (where X is the number of Seraphim squads in the formation).
* * * * *
Mostly took a cue from the Land Raider formation that Black Tempalrs get, and I think it would make sense for a bolt-weapon heavy squad like Seraphim to go with it. I'm iffy on the no-scatter Deep Strike, but at the same time, if I allow them to spread out, that means the Rain attack could be very wide-reaching indeed.