All Daemon Core is WS5 T3 so Saurus pound on them when they hit, but the Unbreakable and 5+ Ward save across the Army make them take the pounding and stick around a lot long than you would like.
At 800 points things that will give LM trouble:
Flamers - 1d6 S4 flaming shooting attacks each, Skirmish, S5, 2 Wounds, will see a unit of at least 3
Bloodletters - S5, Killing Blow add a Herald - they gain Hatred.
Daemonettes - 2 Attacks, S3, Armour Piercing add a Herald - they gain Always Strike First.
Fiends - Move 10, S4, T4, 4 Attacks, 3 Wounds, Armour Piercing
Khorne Dogs - St4, 2 Attacks, S4, Swift Stride
Things that can hurt Daemons:
Light magic, but they have a banner for that. It is 50 pointer so not sure you will see it at 800 points. But at 800 points you can not bring a Slann so the point is moot.
They all have high I so things like Purple Sun are only fair/to poor against them. But at 800 points you can not bring a Slann so again the point is moot.
Salamanders can not panic them but they can roast a lot of them at T3. Razordons maybe a good choice here but that is untested.
Mayby go with a big SKROX unit, sure they are going to execute a bunch of Skinks but they can not Killing Blow the KROX.