I want to remove an overgrown forsythia that is a "foundation planting" near our old farmhouse.

It is huge, it is nasty. It is planted among the barnstone terraces and retaining walls.

My previous experience trying to kill one of these beasts was not an unqualified victory.

I want to put a nice lilac in that spot in the spring.

I'll cut it back with the blade attachment to my Echo trimmer. But how to kill the stump and roots?

This is also fairly close to my wellhead.