Best way to build my defences on Mercenary?

Currently playing gemling legionaire, and I'm very new poe2. Most guides gloss over defenses, and I can't find a general consensus on what I should be running. If it helps at all, I'm playing a lightning focused build, galvanic, plasma burst, etc.

What armor stats should I be looking for? I've been able to get by, but going into endgame I have no clue how I should be building my character.