I'm looking for some suggestions on optimizing some of the monsters to fight my party. In this case, the (5th level) PCs have been cutting through hordes of lizardfolk (of various types) with ease. The lizardfolk have had some opportunity to learn from their encounters, and they are aware of the following:
  • The PCs (mostly) have excellent ACs.
  • The PCs have a lot of healing magic, and downed foes don't stay downed.
  • The PCs have a lot of area effect magic, so grouping up is a bad idea.

While the standard Lizardfolk can't really alter their plans much beyond making heavier use of javelins instead of packing in for melee, the spellcasters are a different story.
What are some alterations to the spell lists for the Lizardfolk Shaman (Druid 5) and Lizardfolk Subchief (Cleric 5) that would make them more deadly foes?
I've considered having the Shamans use moonbeam and the Subchiefs use spirit guardians for some damage that ignores AC. It's also a benefit that moonbeam can wreck the PC Druid's wild shape rampages.