The DK patrols do take a little strategy to defeat, at least if you want to fight fair.
What type of weapons are you using, any what character types are your hero and DK?
For conventional attacks you will need to have a reasonably good Agility (accuracy and evasion), Survival (initiative and accuracy) and/or Speed (evasion, initiative and attack / recovery rate). For magical attacks, they are not particularly vulnerable to anything. Resistances to conventional attacks are all 10, bone and shadow resistances are 30, while everything else is between 50 and 60.
It was at this point in the game I switched my hero to a full time archer (set to aggressive, to automatically attack if not selected as the leader). Before that many of the opponents were better dealt with using crushing damage, and I was conserving arrows (not necessary with the patch installed or an already updated re-release game version). In this area I moved both characters separately, to make sure the archer was always following behind, so that only the warrior would attract a DK patrol.
BTW, playing again I would have the DK be an archer (bow). Since his armour goes up with his level he doesn't need as much strength for equipment requirements, and could concentrate a little more on agility.
When sneaking you should still be able to outrun the DK patrols, and get to a storage room, etc. Hit and run tactics work well (especially if you sneak in, use a ranged attack and withdraw before they can see you). You can have one character run around using hit and run tactics, or simply as a diversion, while the other does most or all of the damage from a distance (bow or magic, though with a bow they can be set to aggressive to attack continuously).
The act 1 summoning doll can equip a crossbow. It may not last very long if attacked, but if you keep it out of harm's way it can provide good support. Most of the time I didn't bother with it, but it came in handy for a few tough fights.
With the latest game version the BF merchants regenerate their inventories each time you enter, so you could try visiting a few times to get better equipment.
