Help with my fire blessing's team 4th unit;

I'm building teams for each of the blessings, and so far the fire team is this;

1. Sanaki: +RES -HP, 30/51/26/17/37
Cymbeline (+Effect), Rally Defense, Iceberg, Atk/Res Bond 3, QR 3, Hone Attack 3, Drive Defense 2.

2. Gerome: 50/55/28/41/22
Slaying Axe (+Def), Reposition, Ignis, Fury 3, Axebreaker 3, Drive Res, Guidance.

3. Legendary Ephraim:+Res -HP, 40/55/27/33/21
Flame Seigmund, Ardent Sacrifice, Sol, Sturdy Stance 2, Solar Brace, Fortify Def 3, Heavy Blade 3

Sanaki doesn't get much out of the def boon, besides being a bit better able to tank the dragons. She can tank Reinhardt when she's fully buffed & on a defense tile, so I figure there won't be much else in the way of magic attacks that can kill my team. She's partnered with Gerome.

My problem is, I can't decide on a third option. I'd like it to be armor or infantry, so as to use guidance, but I'm not sure what would work best with this team. What does this team need? What would this team struggle with? And what are some suggestions for units to fill it? (I have a pretty wide swath of units, even if I don't have one I'll probably have something similar.)