Just bring a high meele doed on the city at extinction and hit the lamp post for dust. Each 1000 dusts give 1 element. Do take note you can't transfer element, so if you're not from extinction, upload the dust and craft the element on your base.
On crystal isles, check around the bubbles on monster island and u still dozens of crystals and they give you element. Also check up near top, there is a huge crystal forest that gives elemt. And make sure you hit the small crystals, not the ones that zap you.
As said by others, element can be mines on specific asteroids on the Genesis 2 map. It’s pretty distinctive. The asteroids have an almost ascendant teal type shade to them, and the have basically glowing ascendant teal crystals. The crystals drop primarily Shards and Stone, with a small amount of element even on boosted resource single player game.
An Ankylo is you best best for collecting large amounts of Element. A melee buffed Mantis with high level picks may be able to do better, I’m not sure.