use your pally. you will need to go to DM east first to chase down the IMP for the DM west key.
in DM west, you will need to deactivate all five pillars that keep the center chamber locked. also, you will have to face one tree boss and a demon bulldog looking like thing, which is the final boss in the instance. make sure you kill him off of the platform since you will like to have the center platform clear for the summoning.
defeat the summoned demon and the dreadsteed.
boot the pally from your party and invite your fourth lock.

as long as the defeated dreadsteed is still spawned, he'll be there for a while, any lock who's at that stage of the chainquest can talk to it and learn the summon dreadsteed spell.

oh and one thing about the fight... drop all your demons and just enslave the felguards that spawn. they will he super helpful, and it takes away 3 or 4 mobs you have to kill.