How to beat Robin GBH with armored team WITHOUT Tharja?

Wait for PheonixMaster1 to upload the video. :p

Is it Hector (LA)?, I Managed to beat infernal by having him with vantage 3 whilst supported by a hone armor from a teammate and an A support, if not res 3 seal could be useful.

Amelia can definitely be kept since armor march would be needed.

Any mage could be very helpful, e.g. Lyn (Love Abounds) since you'll need to kill off one of the green infantry, recommended the one bottom of the map,
This would make the female robin attack Amelia if you placed her correctly when swapping at the beginning and would allow your DC Hector to one shot her if i'm correct.
Essentially you could do this just by using 3 characters, Hector or BK, and Amelia is important

Hard to say without knowing specifics, but Atk Smoke & ploy seals really helped out for me

I did it with regular LA!Lyn (she just killed the red mage so if your DC Effie can ORKO that mage in the enemy phase, use her), Hector, W!Robin (just used him for Armor March, you can use Amelia), BK (he needs Escape Route 2 and the Distant Defense 3 or the HP+3 seal, also at least B-support with Hector I think, mine are S-supported). In those conditions, my BK was able to survive the blue mage, kill Robin and both axe users. He lives on very low hp so he needs Escape Route to get away. If the AI works the same (which I kinda doubt), in the next turn, you have to take everyone out of the danger zone put Effie next to the upper wall so that she can tank and kill the red mage. After that, Hector can take care of the lancer and the blue mage

But again, the AI has to function the same way, which I kinda doubt it will

I used BK with Vantage and Dist Def seal. The important thing in my run is that he must have pivot so he can fling himself forward towards the tile to the left of the water tile in the middle. The unit in front of him before he pivots must have one of the field armor buffs and another armor next to him (north or south) must have the other field buff. The 4th armor unit must then carry Ward Armor, who must be in the tile where BK was before pivot. The importance of this setup is that he gets all 4 buffs from hone and fortify and he can melt the opposition.

This way, he destroys Robin and both axe units. I also had Aether on him so he can tank one of the remaining mages and destroy them. I used Sheena (who carries fortify armor by default) who is an excellent wall at the choke point with a defensive spread of 40/37 def/res. She should have no problem dealing with the blue lance unit as well as fending off the red and blue mages. I had Slaying Lance res refine on her though.

Took a beating but hector the mvp

Well I don't have W!Tharja and managed.
I used LA Hector, but I think regular can pull it of if you give him wary fighter.
Basically I put him next to sheena to get fortify. Then I put him on the front row with Amelia behind giving him armor march and a unit behind with ward armor.
After the first round most of the ennemies are pretty well damage and you should be fine for the rest of the game.

Black Knight. Buff him up and park him in the range of the two axe users. Give him DD seal (mine had QR2 but did nothing since he outsped most enemies with Hone buff), he will KO half the enemies on the map. Only thing to watch out for is the blue mage. With Amelia's Armor March, you can pull away quickly if you need to. There's no enemies casting panic so make use of Hone/Forts.

I took my +Def Halloween Henry, had him receive Fortify and two Ward armor buffs, and his support buffs, and he could easily tank all of the units on the map

The key is armor buffs. I followed PM1's guide (I used W!Chrom instead of Sheena) and it worked out just fine.