Berkut

This is my current Berkut build that I have invested for since I've heard that IS was doing GHB Revivals.

*Weapon - Berkut's Lance+ (+RES)
So when I was originally looking that multiple builds from Gamepress to get some ideas, I noticed how they added a +SPD refinement to Berkut DC build. As they mentioned how doing this would help him get his speed high enough to double with Calvary buffs. But in my opinion, I don't think your going to get as much out of him because of the positioning requirements needed to get all of these buffs. As I chose to give him a RES refine instead to balance his bulk against mages and archers for a purely focus enemy phased build.

*Assist - Repositon/Swap
You can choose either of these skills as they can save potential back line mages and archers on your team from strong red or blue melee threats.

*Special - Aether/Bonfire
I personally use Aether as it is good for Tempest Trials and Chain Challenges, as it provides him the ability to kill high DEF tanks easier as well as giving him a way to heal himself for QR.
As for Bonfire, it is more of a budget choice but it does have its perk of being more consistent than Aether.

*A Slot - Distant Counter
This is the bread and butter of this build as it allows Berkut to have a bigger impact on Enemy Phase. Allowing him to be a threat to both
mages and archer (besides Bow Lyn ;-;).

*B Slot - Quick Riposte 2/3
QR help Berkut immensely to make up for how bad his speed is. As being able to attack twice helps him either kill his target, or charge up his special. As he get 4 charges when he is attacked at 70% health or higher because of him getting doubled as well as QR activating. As this would help allow Aether to proc in the next Enemy Phase he is in.

*C Slot - (Situational) Hones, Fortifies, Spurs, Etc
This is total dependable on what your team is. As me personally, I've put him on a Horse Emblem team which means that I've gave him either Ward Calvary or Hone Calvary for the others Horses I have around him (Sigurd, Bow Lyn, and LA Lilina). But if you're running a mixed team, you can choose to put hones or other buffs that any Units could use.

*Seal - Distant Defense/Close Defense
I use Distant Def personal to enhance his ability to totally deny range threats in general. But you can use Close Def if you worry about his melee match ups against other lance and sword Units that are in the game. As these seals are important for Berkut to take as little damage as possible to stay in QR range.

Note: all stats include skills because I'm lazy

Berkut's Lance makes him into a great magic tank, with Fury only adding upon that. Lancebeaker so he can actually double something, with Hone Cavalry to make his allies stronger. Astra turns him into a +100 damage nuke Savage Blow to deal some damage to his surrounding enemies. Finally, Reposition lets him move his squishier allies behind him.

This set makes Berkut into an excellent Dragon Check, due to his high stacking resistance and slow speed, guaranteeing an Iceberg activation should his foe double him due to Quick Riposte. Close Defence also allows him to survive most red and some blue melee units, having high enough defence to live through their attack and kill using Iceberg combined with his High Attack. Close Defence seal can also be run, increasing his overall bulk and Iceberg damage (when attacked from 1 range).

Berkut may have a respectable atk skill but with such a low spd, he can't put it to good use. In order to get any doubles in at all, this specialised build with the Desperation and Brash Assault combo allows him to be somewhat impressive, under certain ideal circumstances. With Fury to boost his atk further and also to make it easier to get his health down to 50% along with Reciprocal Aid, he can attack twice in a row at full strength without giving his opponent a chance to counter. He has Bonfire to take advantage of his def, although Luna could be substituted. Because his goal is to be able to attack twice to take out his opponent before they can counterattack, Slaying Lance gives him a greater chance of activating his special and ensuring his survival. His C skill is flexible, but Threaten Def is helpful in ensuring that he can kill his opponent before they have to opportunity to double him after his health has been lowered in order to activate his skills. When he is in top form, he can take out a Black Knight even if Black Luna is fully charged or a Triangle Adept Performing Azura. Even Lukas cannot escape his deadly OHKO. His main weaknesses are Firesweep weapons, Lancebreaker, and archers with high def, as if his foe cannot counter and if he is prevented from attacking twice, his combo is useless. There is also the fact that in order to use Brash Assault, his health must be low, leaving him vulnerable, but his high mobility and decent def make it manageable with proper support.

This Berkut works well for those that don't have distant counter fodder and want him to serve as a mixed one-range tank. With all of the dragons in the current meta, Berkut is actually quite capable of taking them, as well as lance and sword users. This build capitalizes on Berkut's low speed, and he functions similarly to a Lukas that can tank dragons and has one extra movement (and can receive cavalry buffs). Basically, you want to stick Berkut in front of either a blue or red melee unit, which Berkut will usually kill after he doubles them, due to his high attack. Then, you'll have Ignis charged, and you can fire it off for a high amount of damage onto an enemy unit. Bonfire can be switched out for Ignis for a better enemy phase, as it will guarantee more kills.