my defense team: Pitiful or Unbeatable

You will never know how many losses you have, since the game only show you your wins in arena defense. If nothing appears on your defense results, chances are likely a ton of people beated you, or nobody bothered to fight you. Your team isn't bad, but it's not unbetable by a long shot.

All ur infantry units are easy to take down/dont hit hard enough/easy to manipulate.
Blyn is good but shell tend to rush and be left separated making for easy to tank and finish off. A big part of why shes threatening, is that shes hard to bait out because of her fellow allies.

Your team needs something to really put it over the top.

Gimmicky units/skills, synergy, something that will catch your opponent off guard. That is what you really need. If you can’t do that, then there will be weeks you will get 0 defends.

It's pretty easy for my current team.

My physically defensive Steady Breath Nowi would lure out Brave Lynn and get the easy kill thanks to -Res. Then, I'd clean up L!Ike with my Nowi, Shiro with my Myrrh, and B!Ike with my Alm. If somehow L!Ike beat Nowi (which isn't really possible), Alm could just finish him off. If Myrrh died, Nowi can beat Shiro. If Alm died, that could be tricky, but Myrrh could probably deal with it with the help of my fourth team member.

I think that Brave Lynn is amazing for early defense, but loses a bit of value later on because everyone carries a counter to her, meaning that she can just get lured and die. Sure it's guaranteed heavy damage, but I don't know if that's necessarily more valuable than another member.

I have more trouble with hyper-bulked-up armor teams.

Defense teams require more than just good bst and units, they also require your units to do something unpredictable in order to catch the player off guard and make then mess up.

from the little information we gave me, I would just throw my 56 def Lucas at that team and watch as he true solos.
Having diverse units will accommodate that, maybe replace Shiro with Nowi

My current defense team is QR, ignis, Infantry pulse, 56 def Lucas, Warding breath, Aether, QR Nowi. Firesweep, galeforce, L&D, Guidance Cordelia. And a Divine Naga, fury, Escape route, res/atk ploy Julia.
with this team I cover 2 physical units and 2 magic units, have both a def and res wall and debuffer, the guidance on Cordelia and Escape route on Julia makes their movements almost impossible to predict. Plus Julia is built to take full advantage of the maps to debuff their tanks and one shot any dragon unit. Each units weakness is covered by another unit, and having three of them be blue make its insanely hard for redlords to make their mark.

Defense teams require more than just good bst and units, they also require your units to do something unpredictable in order to catch the player off guard and make then mess up.

from the little information we gave me, I would just throw my 56 def Lucas at that team and watch as he true solos.
Having diverse units will accommodate that, maybe replace Shiro with Nowi

My current defense team is QR, ignis, Infantry pulse, 56 def Lucas, Warding breath, Aether, QR Nowi. Firesweep, galeforce, L&D, Guidance Cordelia. And a Divine Naga, fury, Escape route, res/atk ploy Julia.
with this team I cover 2 physical units and 2 magic units, have both a def and res wall and debuffer, the guidance on Cordelia and Escape route on Julia makes their movements almost impossible to predict. Plus Julia is built to take full advantage of the maps to debuff their tanks and one shot any dragon unit. Each units weakness is covered by another unit, and having three of them be blue make its insanely hard for redlords to make their mark.

Its okay but the computer controls B!Lyn really poorly. Arena is easily gamed because units move without being aggro'd. Just place a unit close enough that B!lyn will rush towards them and then kill her turn 2. B!Ike is also unfortunately poorly AI controlled, I run a B!Ike on my offense team and its hilarious because whichever B!Ike attacks first loses and AI will always attack.

I would switch your B!Lyn with your Performing Azura. I've found its hard to predict what the AI will do with a dancer, they don't always do the right thing but they can suprise you and catch a key unit out that screws up someone's team/plan and grab you a win.

What tier are u in? Sometimes your team is op for the tier you are in.