I found it was all about protecting the access to the heal extenders in the top right corner. I merged the zomblin gravestones as quickly as possible as I killed them and placed the first one next to the one leading to the heal extender.
I got the broken gaia statues close to each other on that part of the grid so I could move quickly when I merged the next lot of gravestones.
With the second merged headstone, I could use the heal extender, free up the gaia statues and get them merged.
It helped to merge the red and blue eggs first, creating the space to merge the 5 available spotty eggs.