There are a few things about merasmus which only work on lakeside_event (aka ghost fort). His spawning as well as some of his attacks are hardcoded into the map.
The only things that can be replicated are:
Spawning Merasmus (requires his point entity "merasmus" as well as a point_template)
Having Merasmus kill people with his weapon thingy (happens automaticaly)
Merasmus starting to hide after taking certain % damage (requires nav mesh and the boss_nav_prefer mentioned before)
People getting bombheads (bombheads won't actually do shit from my experience)
People teleporting to certein places after he gets defeated (requires info_target entities with very specific names)
He will never start floating around in the middle of the map and throw bombs everywhere. Even when using all entities that could be required for it, it simply wont happen.
If you want him to be usable in hammer, download and install this fgd:
https://www.dropbox.com/s/axohlefk8ky4a45/halloween_bosses.fgd?dl=0
Place point entity "meramus"
Give targetname
Place point entity "point_template"
Give targetname and put in merasmus for "Template 1"
Make output somewhere that tells the point_template to "ForceSpawn"