Tactical positioning.
I recently had a hard time in the mountain pass with a slightly under-levelled party versus reanimating undead. I had hacked them down to just two before a series of bad RNG and were outnumbered again - and losing!
We withdrew, leaving behind a sacrificial skeleton as rearguard. This split the enemy with some pursuing, the remainder ganking the rearguard.
I took the initial pursuers down and leveraged my numbers to cripple key undead. Downed undead reanimate at full health, but crippled undead stay crippled. Then in one round I overwhelmed the 2 key undead so there couldn't be more reanimating. From there it was destroy in detail.
Recognise when situation is changing, and reposition for a more limited goal. Leverage an advantage to snowball into overwhelming odds.
Also, Withers is a resource, use him. You can raise the fallen, most foes can't. Yes, this is brutal and cynical, but victory needs no explanation.
