Goodness...

https://www.youtube.com/watch?v=_gz-KLV1Yjw&t=319s Watch this video from Pheonixmaster. He used only a four star Legion, Ursula and Olivia along with a five star Sharena to beat Berkut on Infernal

If you of any unit that can sub for camus you can get him in the bottom left corner defense tile with a none cav buff and he will on infernal will almost take out 3 units have have his special charged. I honestly don't use really any units than my main team so I couldn't be the largest of help 2 you but if you have a complete horse emblem team you can beat lunatic THOUGH it's extra hard on infernal. I literally just several videos how and saw each role they played, Id known I WOULDN'T have the same units but I did know I likely could have some that could do that same job. It took me several tries and a couple of potions but you'll get it and once you it it becomes easy.

You seem like you can make a strong cavalry team with Xander and especially Olwen. With hone cavalry from Xander, Olwen should be able to blitzkrieg many of the units. You could add a dancer like Olivia for support to let Olwen kill 2 units in 1 turn or retreat, and perhaps Anna.

The teams posted here are only for the rich or the lucky ones anyway.

Check out Mkv., nstyler, or ItchyBackside's channel on youtube. They regularly post how to clear GHB with free, vanilla 3-4 star, neutral IV units.

Lunatic:
https://www.youtube.com/watch?v=Jk0klTZO-7o
or
https://www.youtube.com/watch?v=mZnm-eb-J2o

Infernal:
https://www.youtube.com/watch?v=dYkJB9ucahc
or
https://www.youtube.com/watch?v=kzIpcMNbQXA&t=3s

My Sharena is already 5*, so I equipped her Silver Lance and Speed+2, and it still works. Olivia 4* is fine too.

Sorry you're having trouble.

This GHB is indeed pretty tough. I also had trouble killing even one unit in Lunatic with my regular teams (horse emblem, Nino team).

Considering Tempest is still running I got lazy wasting stamina just to try teams.
So I've wandered through Youtube to find "F2P berkut lunatic / infernal".

There are many options, and the data pool is growing everyday. I'm sure you'll find at least one of them that will fit your needs and army.
You don't even need fully built characters !

I beat the two modes
Infernal with Nino, Palla, Olivia, Narcian
Lunatic with Xander, Olivia, Ursula, FRobin

I'm sure you'll end up beating it. Good luck !

Hey!

I just beat infernal with Nino, Delthea, and two Olivias. I think you could probably recreate it with Julia, Linde, and two dance/singers of your choice.

I started with Delthea across from Berkut, Nino below him. Nino one-shots the blue cav. Olivia1 sings to her, Nino moves out of range beside her. Dethea moves up to the defensive tile and Olivia2 moves beside her into the top right corner of the map.

next, Delthea attacks Berkut. Olivia2 moves below Delthea and dances. Delthea kills Berkut. Nino can now kill the archer with Olivia1's help with movement.

I kinda forget what happens next, but I think I had Delthea oneshot the red mage, Nino oneshot the green axe, then I danced them to kill the last unit.

hope this helps!

You have some very good units and should be able to manage this, particularly if you happen to have access to Lancebreaker and Bowbreaker anywhere in your reserve of units (you don't need anything higher than level 1, 2 if your unit is running Fury).

I was able to clear Infernal with Tharja, Bowbreaker Klein, Lancebreaker Julia, and a 4* WoM Olivia. You could replace Klein with Olwen or Linde for a similar approach and use Xander in place of Tharja. I wrote up my strategy on the GHB page, so you may want to take a look there. In your case, rather than bait an actual hit from the red mage, I would imagine that Xander should be able to use his movement advantage to bait him over and then kill on initiation. Alternatively, Xander can come down to face the green units as they move across the page while Olwen comes up to meet the red mage.