i killed them in like round 15-20 only way to deal with them

If you play Empire its a good idea to send at least 1 lord east early (after you secure home province) to help the Electors not get overwhelmed.

The "Return to Elector" decision on capture of settlement is very powerful and you can quickly return multiples cities or resurrect Electors. In my game it was enough so the Undead get beaten back while I conquer Brettonia with my main force. Its safer to go after Manfred with all you got but I prefer letting them live for a while and returning cities for faster Elector relations and Authority.