That’s just the end game crisis, or you’re playing Vortex and that happens at set points (when you do a Ritual).
Thing to understand with Chaos is while they absolutely shred in Melee and are heavily armored, their ranged component is middling at best. So for best results, just tear them apart with Artillery/Ranged and Spells.
For Vanilla High Elves - Eagle Claw Bolt Throwers, Archers, and a Mage of your choosing to spam their vortex spell (Avoid Fire, though. Some Chaos Lords get resistance to Fire Damage).
For DLC High Elves - Sisters of Averlorn. Sisters of Averlorn just MELT Chaos Armies.
For Dark Elves, both Vanilla and DLC - Darkshards/Shades with Great Weapons. Dark Elves handle Chaos pretty easily, honestly.
For Lizardmen, both Vanilla and DLC - Stegadons with a Slann Life Mage
For Skaven in Vanilla - A full stack of Skavenslaves to tar pit Chaos Forces and get aggressive micro-ing with Doomwheels (Always keep them moving).
Skaven with DLC - Ratling Guns, Warplock Jezzals, and Doom Rockets