Well, duh. I mostly run story based PnP with very few fights, but when there is a fight it is as deadly as realistically possible given the rules system used. Such as if I am going to have a hostile Duergar in the encounter he is going to use his full capabilities to kill his adversay, or to run away if the fight turns against him. Duergar have a number of special abilities and they would have used them in that fight. If Obsidian didn't want the duergar to use their abiliities then they should not have used duergar. Goblins or kobolds would have been a better choice as fodder for the githyanki and the bladelings, but Obsidian choose Duergar therefore the Duergar should have used their full strength and potential otherwise that is just piss poor design.
Of course some battles can be planned to be the most difficult fight for the PCs lives until a PC makes up a brilliant plan and gets a lucky critical in the first round. Let the die land where it may, you know.