What is the most terrifying unit you ran into in arena

Not sure, to be honest. A lot of the common units tend to use the same builds, and the more "creative" units are fairly easy for me to get around provided I have the right team.

I think the most "terrifying" unit I've encountered was that Bold Fighter armored lancer from Julius' GHB. lol

+10 MGrima with a +10 TA Ninian behind him on that arena map with the 4 bridges connected to a small center island. My ATiki would have been killed by the Ninian and my LAHector got killed by the Grima.

A Grima running a defense refined Dark Breath with Distant Counter, Bold Fighter and the quick riposte seal. Now, that Grima had a L!Ephraim supporting him as well. He was hitting 58 attack and 43 defense before buffs, so it was damn near impossible to kill him. Surprising how changing one thing can make a unit that much more terrifying just because they had 4 more defense.

While I don't see her very often anymore, I always fear the Horse-buffed Brave Bow+ B!Lyn +10 because no one on my team can live her attacks (without being on any defensive platforms). She will almost always kill my Hana and Nino in her two shots, and quad my Nowi, often activating her Special in the process and killing my unit.

This sort of counts as “a” terrifying unit. I ran into a team of 5*+10 Merric during my deathless arena run for my final match, while using my flier emblem team.... They were all built completely differently, very cool. They obviously had a thing for him and put a lot of hard work and dedication into building all 4.

It was probably a build similar to that but on M!Grima

This was in AA, but LA!Hector with Vantage and Ignis. I'd hit him with attack ploy, but Wrath-boosted Ignis does not care, and he OHKOed my Xander from full health. Ouch.

Here are the most terrifying units I've faced just recently. Lyn and Celica were on the same team. I beat lyn and my +6 Soren barely lost to Celica after taking 11 damage from a QP+Moonbow Reinhardt.

Edit; Not even after a Chill Atk debuff. My Soren would have survived if he at least had one more ally beside him. He was only beside Amelia with Spur Def/Res.

My winter Lissa is running with something similar, she just does not have Warding Breath coz she needs DC there, but she still hits like a freaking truck.