We do a very similar strat to yours, OP. We have WAR OT take soldier/knight/bugs while DPS LBs the rook then immediately burn down the "old" Dreadnaught because our biggest problem was the MT taking too much damage from 2x Dreads. Since your problem is the OT dying, I'd say either let him use tank LB or have dps get on at least one of the adds before going after the old dread, if you think your MT can handle it.