Hahaha this reminds me of a fun game a week ago. An eldar player used phantasm and redeployed his rev titan on the other side of the board (we were playing a 3 table split deployment with imperials defending 2 separate tables) against the smaller table.
Well this meant that there was virtually nothing to take on the titan on that side of the board so I moved my forge world destroyer (named "the emperor's light) 6" forwards and attempted a hail mary shot against the rear of the titan. I was barely in range of the titan and rolled a hit on my measly guard BS. What happened next was amazing. He failed his titan fields save and I rolled on the damage chart. A 6. Reduced to 2 structure points I rolled again. Another 6. All of us laughing (at least on the imperial side) I rolled again. another 6. Now even my opponents joined in (all on his side except the one owning the titan) Then for the grand finale I rolled another 6 and caused an apocalyptic explosion, killing a total of 9 other CSM (the other team members) and stunning a dreadnought.
Sorry but the mere mention of a revenant titan always brings this to mind
But back to your topic; the Phoenix court of Kane can be useful in that only daemonhunter weapons and large anti-tank weapons can generally harm the avatar since in that formation he's eternal warrior and immune to plasma as well. And all the phoenix lords are great in combat so it is quite powerful and I don't think there's anything against them leading their own aspects anyway (although i'd avoid that with Maugna Ra as that would be a waste of dark reapers)
I think the striking scorpion formations is one of the best ones out there. They can charge into terrain and wipe out any enemy only to move all the way across the board in the next turn. Very useful on the large apoc boards. although the warpspider one is pretty cool too.
The wave serpent one is sweet as well, forcing large numbers of pinning checks on its way to deliver its pay load. I've had to play against this, and it is very annoying for my guard