How can you work with Zelgius' low res?

Maybe try keeping him away from mages? And let, you know, actual magic tanks take them on?

If you run him on an armor team just run a few wars armors and keep one or Two armors next to you if he MUST block mages. Thing is, it’s prefered you just wait it out and player phase mages, if you can’t then run wards or drives if you’re on a mixed team, or let another unit kill the mage.

Warding breath?

I run my zelgius with warding breath and distance def 3 for his seal. I also run vengeful fighter.

The easiest solution is to run him with an armor emblem team, to get fortify and ward buffs. Otherwise, IV's, merges, and supports are something to look into. I personally run Warding Breath on my Zelgius for extra res and Aether to heal back up damage that is lost. You should avoid most blue magic wherever possible, and against Reinhardt or Micaiah he stands no chance.

I use warding breath and 3 ward armor's which gives him
52 def and 38 res

One-hit by any mage? You have like three HP banes or something? No mage that doesn't have a Lilina-like atk stat, effective damage like Micaiah, or a fully buffed blade tome should be able to down him in one hit. Even Reinhardt should need both strikes of his thunder if Zelgius is at 100% HP unless hone cavalry is on.

If you want to boost his anti-mage potential, you can go warding breath and distant def and stack ward armour on him like the others said. I would suggest accepting that he shouldn't fight non-green mages (and against green mages he's looking for a OHKO, so don't let Nino initiate if her desperation is active). I've used the guy a fair amount myself with +res -HP.

Pardon if I seem a bit cranky. Was trying to offer some analysis on the release of the new units' stats and my machine crashed, destroying my work multiple times.

You shouldnt put zelgius vs any mage at the first place, but if u insist on tanking and counter-killing a mage, warding breath with deflect magic 3 seal might work.