If you are playing the OC and you just can't disarm it, then either bash it with a missle weapon as above, or recruit Tomi Undergallows to do it, then go back to what ever the safe haven portal location for which ever chapter you are doing and drop him off. If the chest turns out to be locked too, then he can unlock it once it is disarmed.
If you haven't been using Tomi up to then, have him level up before trying the chest. If they haven't been adventuring along with you as your character levels up, then the henchmen don't level up, but you can initiate conversation and somewhere along the conversation tree there will be the option to tell him to level up (to your characters present level). That way Tomi will have enough skill points to handle a high DC trap and lock.
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