They are a very high level summon, so they are supposed to be very tough to kill!
Best way is to take out their commander, so they retreat. If a mage already Beserked them, that may not work though. Put some riders on each side with hold and attack rear and hope for the best.
You do want to curse them, as a blind Abo is a lot easier to deal with, as is a feebleminded one.
The undead archers are good - tie them up with cheap troops, and accept some friendly fire losses.
MR is 20 base for the Abos, so anything with a magic check is pretty tough. Disintegrate may be your best bet there, but you will need a lot of attempts (or a Spell Focus and a high death mage).
Since they life drain and have very high attack values, you probably never will be able to overwhelm them with cheap living troops. Slow them down with undead or summons so they don't get to regain life. A good number of calvary might be able to take one, but since they are strength 20 and the life draining is armour piercing, they won't Last too long. If you want to pin them, have some death mages raising soulless (more bodies each round), and eventually they will tire themselves out.
Elemental isn't bad - Incinerate will hurt them a bit and has decent range, and Frozen heart and lightning too. Need a couple of mages for that of course.
Also the heat/chill temp effects might be good - if you have some ghosts (they radiate cold) in the back with hold and attack, you can hope that by the time they engage the Abos your Soulless legions will have tired them out, and then the chill effect will keep them from wakening up.
Or get your own Abos
