Wither and DecayFollow

Hello. We managed to tackle the first two raids in The Broken Mirror quickly and easily. The third raid (Wither and Decay) is giving us problems. I am sure our strategy is flawed. Is there anyone who can help me figure this raid out? Here's what we are doing.

1. Burn Wither and Decay to 50%.
2. When they hit 50%, one of them will be prepared to die. We kill that one and try to leave the other at 49%.
3. We wait for the mob we just killed to respawn and burn him down to around 50% then repeat step 2.
4. We kite blobs while doing the above.
5. The problem we run into is that at 20 minutes, several blobs start to spawn and keep spawning wiping us. In addition, we had a combined kill of Wither and Decay of 5 times, yet they still kept spawning. How do you get them to stop spawning and die?