Desperation + Brash Assault Seal

Run Desperation 3 in the B slot and Brash Assault 3 in the Sacred Seal slot, and have their HP at less than 50%. Brash Assault gives an automatic double to your unit if they're fighting a unit that can counterattack. Desperation makes it so that any doubles you get happen immediately, and your opponent can't interrupt. Normally you'd have to have 5 or more higher speed on your unit compared to their opponent to achieve this, but not when you have Brash Assault and less than 50% health. This is a way to make Alfonse somewhat viable, either with Folkvangr or Brave Sword+, getting free quads and maybe a Luna activation.

Brash Assault: If unit's health is below 50% and is attacking a unit that can counterattack preform a follow-up regardless of speed

Desperation: If unit would preform a follow-up and health is below 75% the follow-up attack happens immediately

Brave Sword/Axe/Lance, Dire Thunder: When this unit initiates combat gain an additional attack after every attack

Put it all together if you at are attacking a unit who can counterattack with your Hp below 50% you will attack said unit 4 times before you they can counterattack

Brash Assault allows you to double units that can counterattack... however you need to be able to survive the counter attack. Brash Assault on its own is bad, as surviving the counter attack is unlikely.
Assuming you can double attack, Desperation allows the unit to have the second attack to come right after the first, and causes the enemy to attack last. Desperation is an amazing skill, as it allows frail nukes to nuke safely unless they can't double.
If you combine these two together, you get a great set up- Guaranteed double (from brash assault) with far less risk of dying from the counter thanks to Desperation. (i hope this wasn't too confusing)