I'd stay in SM till they're 45 and then move to Strat. Pop in the back-door and pull all trash while heading to the Scarlet side. Doable in 3 or 4 pulls. You could also head to the Baron but there's a couple of annoying mobs in there plus the MC'ing boss is obviously a pain. You could run out the alts and solo her (no MC) but that takes quite some time.

Scarlet bastion is doable but takes some more time since you have to do smaller pulls. Aggro range from the lowbies is also a problem.

The Undead trash from the backdoor to the Scarlet bastion is the most efficient imo.