|Boon||+ Def||Bane||- Spd|
Hero Build Overview/Explanation
So this website will not let me add skills, but basically, you're going to want to swap ignis out for bonfire when making the character as Gustav's defense stat will boost the power of this immensely. Furthermore, if you pair this with the S support skill defensive breath then you are going to have a fully charged special after the first attack and due to Gustav's axe, this first attack usually won't KO as it is going to be reduced by 75%. He should keep all other skills he comes with otherwise. His assist skill really depends on the context you use him in. Since I pair him with Ascended Fjorm, he has rally Atk/Spd+ in order to maximize his ability to support her, but you can also run positional assists if you believe it would help. Harsh command+ is probably the best all-around option as there are easier ways to cover the movement challenge inherent with armor units. With this set-up, Gustav won't lose to nearly any units, especially if paired with flayn or any other unit that grants a damage reduction.
YOU NEED TO HAVE SKILLS ON GUSTAV THAT INCREASE YOUR SPECIAL COOLDOWN COUNT PER ATTACK AND REDUCE YOUR ENEMY'S. If Gustav doesn't have this then he will lose the matchup against fallen edelgard. This doesn't need to be through his personal skills as long as you pair him up with someone like Lucina or Thorr. The skills you equip on him purely depend on what support characters you have him paired with. There is no best skill set.
Lastly, you should not take a res bonus on Gustav as the biggest close-range threats are all physical attackers like Alm with windsweep or especially Nanna. You can increase it as an ascended trait if you want, but my Gustav regularly wins against Idunn who has effective and magic damage. That is to say, there is currently no speedy dragon doubler with the defense to survive Gustav's attacks but also the attack to KO Gustav.
There currently only exists one effective counter to Gustav and that is Nanna since she nullifies all damage reduction and attacks twice when initiating combat. If you want to prevent this counter specifically then you either need to increase Gustav's defense stat in combat through support allies using close guard 3 or spur def/res or any other skill like this. Or if you find another way for Gustav to get an increase special cooldown count per attack either through an Ally, or through equipping something like special fighter which means he is only going to be counter-attacking once when most units double him, then you should take hardy bearing in the S slot which will prevent the double on Gustav and let you counterattack with a bonfire which is going to KO Nanna unless she is saved through a literal act of God. One final comment is that bonfire is especially important as otherwise, Gustav risks not KO'ing Brave Eirika on the counter-attack which means she will have a fully charged special on turn 2 with no damage reduction which can pretty easily kill Gustav. Also in some ways an unmerged Gustav might be better than a merged Gustav as you want him to get doubled so he can reduce the first hit by 75% allowing him to counterattack and possibly kill on the first hit.