I'm trying to create some boss NPCs and they seem to be able to attack through terrain and monument objects.

I have tried with rocket launcher, grenade launcher and M249. The first two are a bit buggy in general, can't see the rockets and grenades or trails.

But even with the M249, they can just shoot you through anything.

This should be fixed as of 0.3.26.  Although the world models aren't always perfect, the NPC shouldn't be able to shoot through construction, terrain, etc. now.

Great to hear! I'll try out the new version.