The strategy I use with Echinops and the Archespore is to cast Ingi (with Incinerate) and then retreat behind a tree, but staying close enough to see the plant burning. If the plant is out of your sight, it will go underground. If you watch it, it will remain above ground trying to hit you, and it will keep burning. With a little luck you can Incinerate it, find a tree that protects you reasonably well while you heal, and then you can dash in to Incinerate again if you need to. I killed the Archespore the first time I went into the swamp in this game, just using Ingi.The key is not to let it go underground.