You tested it on a map, where there are many unbanishable enemies. It really depends where u play how useful rampart is. In crypt or glintstone there are a lot of enemies, that cannot be banished. In zodias, valley, fester all non bosses or mini bosses can be banished.
"Lesser" doesn't refer to hp or lvl, just the "strength" which isn't something u can measure tho. In shuyal you can banish skeletons and wolfs, which are weaker, while those bigger more powerful guys in armor cannot be banished.
Even that isn't consistent however, because in planar tombs the skeletons cannot be banished either, but only the plants can.
Overall the term "lesser" enemies is kind of confusing, but more or less refers to the weaker enemies you face.
I guess hp can be part of determining if an enemy is lesser, because lesser enemies tend to die faster than enemies that aren't. But even with that, there are exceptions.
The only real way of finding out if an enemy is lesser, is to test it with rampart.