One game of mine when Basium at last turned on me, he had dozens of 50+ xp angels from wiping out technologically backwards civs. It was not pretty but fortunately I was able to hold the tide, with Nox Noctis.
Yeah, those Angels can be tough.
When a living unit dies and returns as an Angel it brings its XP with him. The Mercurians seem to be able to level-up to stronger units quite easily from the Angels with high XP that join them.
Add this to early Iron Weapons and a flock of Angels can be tough.