Slit518
Adventurer
I'm making this encounter a bit tougher since my players, a group of 5 who are all lvl 15 with really good gear, plus a bunch of npc followers with them. As long as the "pilot" is in the suit he's pretty much invulnerable. The suit is air tight and covers him head to toe. I might allow gas and cloud based attacks to effect him since the suit HAS to allow him to breath somehow.
"Powerful" is the idea. This is a boss fight and I want them to work hard for the victory. I just ran it's stat block through a DM app and it calculated it's CR at 26.
The suit could have a function similar to a gas mask, which allows people to breathe in hostile environments.
In fact, make the user immune to Poison & Disease as well.