For FK Hard, the build for Gnut needs to have suitable speed & accuracy, but far more important than the builds on any of your team is the composition of the team itself.

The higher you climb FK Hard, the most succesful strategy I know of is using high speed Ally Attackers with A1 multi-hit Freezers.  Which Ally Attackers do you have and how fast are they?

Thanks Wiley, I have 2 Fatmans and Morag. My fastest Fatman is 295. Morag was built for damage, so only169, but I'm not opposed to changing that.

Fk h10 stats are pretty tough. I use razel, 2x fatman, gnut, and yakarl - all above 300spd, with razel at like 350.

Here is a thread from 2 yrs ago where Kram and I both posted speeds and team comps.  There was a bit of back and forth as I was actually in process of fine-tuning my FK H10 team at the time.  You may find it useful.

https://forum.plarium.com/raid-shadow-legends/674_game-discussion/1696097_need-some-help-adjusting-my-fk-hard-10-team/

Back then, I ran a Double Ally Attack with Stag Knight for Dec Speed and 2X Gnut for Freeze, and my speeds were lower than they are now.  After I fused Mikage, I switched things up to a Triple Ally Attack.  These are my current speeds and turn order. This team is 100% win rate in 40-50 turns at 2:10-2:30.  These speeds include +20 from Live Arena Great Hall Bonuses: 

Padraig (Team Lead for Speed Aura): 418

Mikage: 361

Gnut #1: 316  

Fahrakin: 303

Gnut #2: 286

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Gnut #1 is intentionally faster than Fahrakin so that Fyro's Shield is down before Fahrakin's first Ally Attack, which allows all of the A1 Freezes to drop Fyro's Turn Meter.  With this setup, Fyro does not typically take a turn.