NOTE: Some skills and seals may not be readily available to some players, so adjust accordingly.

This build is heavily focused on using Byleth during an enemy phase. Right off the bat, Byleth comes with some decent stats and a few invaluable abilities. He comes equipped with his weapon, the Creator Sword, which allows him prevent enemies disrupting the patterns of follow-up attacks and trigger his special more frequently. His special, Ruptured Sky can deal additional damage to not only dragons, but against beast units as well. Additionally in his base kit, Byleth can learn Distant Counter which allows him to counter units who have tomes, staves, daggers, or bows. He can also learn other skills such as Chill Spd and Odd Spd Wave, which are useful in their own rights.

However, Byleth's biggest flaw is his base Res stat, which isn't spectacular compared to some other sword wielding contemporaries. Because of this, he's more susceptible to staff units who can prevent counterattacks, tomes, and (ironically) dragons. Not only that, the abundance of weapons and skills granting "If unit initiates combat, grants + Atk/Spd during combat" (most infamously Swift Sparrow) amplify this a bit more. If this is a constant issue, luckily this can be remedied (for the most part.)

For his A slot, have Distant Counter at the very least! Otherwise, if Distant Ward is available, this can be a decent alternative. However, if Distant Ward is equipped, he cannot counter any bow or dagger unit unlike Distant Counter. This is purely up to your discretion whether you want a +5 Atk/Res bonus against tome/stave/dragon units or not.

Vantage as his B slot complements this build very well, especially during an enemy phase. Since it allows him to attack before the enemy can strike (while at low health), this can potentially save him from a potential KO. Even with this in mind, always make sure to watch his HP whenever it gets low.

Odd Spd Wave as his C slot not only grants him + Spd at the start of every odd numbered turn, it grants the same amount to adjacent allies as well to give him some team support. This can be handy in more competitive modes such as Arena or Aether Raids. Even Spd Wave could be used as an alternative as well. Otherwise, his C slot could be swapped with something else.

Lastly for his Seal slot, a Stance seal is highly recommended, especially if it grants additional Spd/Res during combat. Swift Stance is probably the most optimal in this particular build, since it stacks well with any Spd/Res buffs granted. This really starts to shine if he has a Distant Counter/Ward + Odd/Even Spd Wave combo since it improves his enemy phase potential both offensively and defensively. If Swift Stance isn't available, Warding Stance or Darting Stance can be helpful as well. However, picking between Warding or Darting Stance can be a tough choice to make, so adjust accordingly.

Handy tips: Thanks to the low cooldown of the Creator Sword + Ruptured Sky, it's possible to trigger his special twice in combat! However, this is only given if Byleth has his special ready prior to combat. It may require some prior setup, but when the conditions are right it'll definitely pay off.

Ruptured Sky has a unique property when it comes to dealing damage against dragon and beast units. Since the special calculates damage independently from weapons like Falchion or Naga, it can bypass units that have a Neutralizes "effective against dragons" passive buff. Oddly enough, Ruptured Sky can be inherited to other units.