Well, until your last line, I would have said unit of 10 Warriors put with a Lord (
RO,
WS,
SW) in a Scythe, as one of those appearing, dropping in at rapid fire range kills things quickly. I usually run two of these.
as for Lychguard vs Praetorians. I run a 5-man shieldguard unit with my Warlord quite often. they may be slow, but I have yet to loose the whole unit. Last game I used the blob in they accounted for a Flyrant, a Tervigon & a Trygon, and they have wiped more than double their points off the board in many games (youtube batrep of one in spoiler)
as for Praetorians, I find they work quite well when paired with a
DLord (
SW/
RO). Assuming the Rods, you then have a very fast unit that hits as hard as wraiths, with only a little less survivability (3++ vs 4+++ & +1T).
so it would come down to preference. I think the
DLord + Praetorians work better for your idea of MobileCrons.
And Annihilation Barge every time. that one isn't even a challenge. you can get two of them for the same price as a barebones SurfLord, and they are a lot more useful in a mobile list (move full 12", you then have to snapshot. Snapshots are 6s. oh, and re-roll all the failures. Cue tesla shenanigans)
hope that helps