Alfonse

This is an odd-turn favored mixed phase build for Alfonse, running Brazen Atk/Def 3 along with his refined weapon, giving him an effective +14 to both his attack and defense during combat so long as his health is below 80%. Odd Attack Wave will boost his attack with his +Atk refined weapon to a huge 59, and combined with QR and Heavy blade, he will almost always have Aether charged after one round of combat. With his HP sufficiently lowered, double Brazen kicks in, allowing Alfonse to almost one hit all but the tankiest blue armors. His only real weakness here is blue armors running fighter skills, such as Hardin, Dragon units with high attack, and Mages, due to his low resistance.

Alphonse is a unit that gets slept on a lot but he becomes a massive powerhouse when given the tools he needs to shine.

Folkvangr refined for Defense. This refine is the perfect option for this build. The extra 4 Defense is more useful than 2 more Attack. Not to mention the fact that his Speed and Resistance are not worth boosting with Divine Dew. It also outshines the Triangle Adept variant because the extra defense Alphonse gets from reaching the HP threshold will allow him to wall out a significant amount of blue units despite the weapon triangle disadvantage.

Ardent Sacrifice (Florina/Rebecca). While Reciprocal Aid could be seen as an alternate choice, I selected Ardent Sacrifice because it gets Alphonse to the Brazen range. While it doesn't quite get to the Vantage range, it gets him close enough so the units that will do damage to him, only chip him closer to its activation.

Dragon Fang (M! Corrin/Sophia). While running double Brazen on Alphonse. He gains +14 Attack before any outside buffs. 65
Attack will give him 32 points of extra damage on activation. This almost ensures he deals massive damage regardless how high the foe's defense stat is.

Brazen Atk/Def (Ares). As previously stated, Alphonse gets 65 Attack from this option regardless of whether or not he is around any allies (possibly more but we'll get into that later). His defense reaches staggering numbers as well at a whopping 50 defense. Capitalizing on his two best stats make this, in my opinion, the single best option for Alphonse's A slot.

Vantage (Lon'qu/Reinhardt). Given that this build wants to turn Alphonse into a mixed phase physical tank, having the first strike is always an amazing ability to have. The boost to his attack makes it much easier to finish off weakened enemies that decide to initiate on enemy phase. Having the tremendous defense of the Brazen skills active boosts his longevity. Those that don't die from the initial counter attack won't be able to make too much of a mark (provided they aren't a dragon). And if you happen to be in the sweet spot between 70-75% HP, Vantage and Quick Riposte nearly ensure the enemy won't be walking away.

Atk Smoke (Kaze). Another skill to increase Alphonse's bulk. Although it only debuffs the enemies surrounding the target, it is still a good option for when Alphonse is going to end up tanking multiple units in the same turn.

Quick Riposte. Alphonse's Speed stat is below average but Quick Riposte makes it so that he is still able to double and increase his damage when not in the Brazen range.

Brazen Atk/Res. In the Halloween Tempest Trial, one of the notable rewards is the Brazen Atk/Res seal. Not only will this bring his Attack up to 72, it will also boost his Resistance to 29, which is a far cry from his below average 22. However, if this isn't as desirable to you as it is to me, Quick Riposte is a solid option regardless.

Being a member of the Askr Trio, Alfonse suffers from many of the same issues as Sharena and Anna, mainly being an inability to have Boons and Bane, and being unable to merge. However, Alfonse can still be a serviceable unit with enough love, who can serve as a nice back-up unit for thing such as Arena Assault, or a last resort Bonus Hero.

This build focuses on keeping Alfonse alive as a Melee Bruiser. The combination of Vantage, Sol, and the Folkvangr'ss effect with a Defense refine make Alfonse extremely difficult to kill with melee, when factoring in his base 32 base defense. Fury, Atk/Def +2, and the defense refine give him a solid 41 Defense, which becomes 48 should he enter Brazen Range.

Alfonse may not be a fantastic unit by any means, but he has very respectable Attack stat. This build allows Alfonse an impressive 56 Attack before buffs, shooting up to 63 when in Brazen range. This is where Vantage and Sol come into play. Alfonse will likely get overwhelmed in places like the Arena due to how pretty much every unit there will outperform him. This build allows him to take some hits, and then use Vantage and Sol to deal some nice damage to the opponent while healing Alfonse for a decent amount, providing him with much more sustainability in combat. Although the alternative of Bonfire or Ignis is also a suitable idea for raw damage output, I prefer the survivability provide by Sol to be sure Alfonse isn't killed off, potentially hurting your Arena Score. Deflect Magic may also seem like a decent idea to prevent his untimely demise at the hand of a surprise mage, this is likely to make little difference in the grand scheme of things, as his pitiful Res will likely result in him getting one-shot before Deflect Magic can even take effect.

Rally Spd/Def+ is far from a necessary skill, almost any other assist will due. I simply used it here because this is what my Alfonse has, so he can assist other units in combat with the combination of a Rally+ and Spur Def/Res, providing an ally with +6 Spd, +9 Def, and +3 Res. Infantry Pulse is a nice alternative for Spur Def/Res is you use an Infantry-heavy team, as Alfonse sports a rather nice HP stat of 43, or 48 with a Folkvangr basic refine.

Weapon: Defense refined Fólkvangr because who needs TA? Our boy needs that defense.
Assist: Honestly this one isn't too important. Change it to suit your needs.
Special: Bonfire or Draconic Aura will work best for this build which will be explained later.
A skill: Brazen Atk/Def because if you aren't stacking double brazen on this beefy boy, what are you doing?
B skill: Vantage. It may seem bland, but all will be explained soon.
C skill: Another variable skill. Odd attack or defense wave are preferable to have Alf buff himself, but if your team composition requires otherwise, go ahead.
Seal: Deflect Melee
Now to the knitty gritty. The purpose of this build is to take advantage of Alfonse's poor speed to activate his brazen stack by initiating into a foe and receiving a double counterattack. Normally, this would be suicide for our boy wonder, but his seal, Deflect Melee, mitigates the damage he receives from the second attack which drops his HP into both Vantage and double Brazen activation. It is of the utmost importance that Alfonse always initiate first, as Deflect Melee only activates during consecutive melee attacks which can only occur during Enemy Phase through the use of brave weapons or weapons with brave effects. If Alfonse is attacked first during Enemy Phase, as opposed to attacking first during Player Phase, his counterattack will interrupt the enemy's attacks from becoming consecutive attacks and he won't benefit from his Deflect Melee seal, thus taking full damage.
Now that Alf is in the Golden Zone, his attack and defense both receive a 14 point boost during combat. This boost greatly increases the damage dealt by either Bonfire or Draconic Aura. The reason either of these skills are used in this build is not only due to their synergistic effect with Alfonse's weapon but also due to their cooldown. With a cooldown of 3, Alfonse will have either of these two specials fully charged during Enemy Phase on the condition that he attacks during Player Phase and that he is doubled during the enemy's counterattack (which isn't difficult at all).
Finally, we arrive at the last integral skill of the build. With both Brazens active and his special fully charged, Alfonse is ready to give his opponent a kick in the teeth should they attack him. Due to his initial chip/chunk damage, his retaliatory blow (boosted by his special and his double brazens) should kill his enemy. Note: one should be wary of engaging any melee foe with hardy bearing and any ranged attacker (as the special's cooldown won't completely charge in one phase if both parties cannot attack). Furthermore, one should not engage dragons with this build as Alfonse's Resistance is still too low.

Wpn: Folkvangr is Alfonses exclusive weapon. Good for arena scoring.
Support skill: Double rally for scoring. Can also be rally Def/Res.
Special: Aether for scoring.
A: Brazen Atk/Def 3 for scoring, and this way alfonse has 65 Atk and 50 Def when he is at 80% Hp or less.
B: Quick Riposte for scoring and for Alfonses first round of combat.
C: Odd Def Wave 3 can help bump Alfonses Def even higher.
SS: Close Def 3 for even more Def.

Since Alfonse is one of the Askr Trio he is available to everyone and a Bonus unit two arena seasons every 2 Months.
Sadly he has no acces to merges. Yet.