Dieck

Weapon: Tiger-Roar Axe

Assist: Positional Assist

Special: Galeforce / Bonfire / Moonbow

Passive A: Atk/Spd Solo / Sturdy Impact / Fury / Death Blow / Atk/Spd Solo / Mirror Impact / Flashing Blade 4 / Heavy Blade 4

Passive B: Desperation / Null Follow-Up / Lull Atk/Def / Lull Spd/Def / Lull Atk/Spd

Passive C: Def Smoke / Time's Pulse / Threat. Atk/Def

Sacred Seal: Heavy Blade / Flashing Blade / Atk/Def Solo / Death Blow / Swift Sparrow 2

• Tiger-Roar Axe's offensive benefits lend themselves gracefully to a more aggressive strategy. In fact, the Axe's additional effects also rewarded for initiating combat; all stats +5 and a guaranteed double attack are great tools to have on a pure attacker.
• Thanks to the cooldown acceleration from Tiger Roar-Axe, Dieck has a few nice Special choices at his disposal.
  • Galeforce's cooldown turn from a bothersome 5 CD to a much more handy 4 CD; thus, with the help of Flashing or Heavy Blade, Dieck can activate it in two consecutive hits (which will occur most of the time with his Axe's guaranteed follow-up effect) and gain an additional action.
  • Bonfire and Moonbow are Dieck's prime choices for burst damage. The former can be supported with a Blade skill or Time's Pulse for single-round activation, whereas the latter requires no skill support at all in return for typically less damage.
• An offensive build, of course, requires offensive supplements in the form of Attack-boosting skill slots. Unless you're opting for Heavy or Flashing Blade, standard skills such as Death Blow, Fury, Solo skills, or Swift Sparrow will definitely work out.
  • For optimized initiation, Sturdy or Mirror Impact will definitely give you the most bang for your buck as it makes matchups safer through added defense and stopping the opponent's follow-up attack.
  • Because Tiger-Roar Axe is able to work in both enemy and player phase, Atk/Spd Solo, Atk/Def Solo, or both are otherwise highly recommended as its stat boosts work in both phases, giving Dieck nice unit flexibility.
  • Note that no matter which skills you pick, Dieck's follow-up potential remains consistent thanks to Tiger-Roar Axe.
• Desperation provides general gameplay usefulness and safety in one package; once Dieck below enough health, he can make consecutive follow-up attacks before the opponent can counterattack. His default Null Follow-Up, however, is also useful in cases where the enemy has artificial follow-up manipulation (i.e. Wary Fighter) of their own.
  • If you're worried about activating Heavy or Flashing Blade, a respective Lull skill will help you ease stat comparisons in Dieck's favor. 
• If not using Time's Pulse, Dieck's C slot is generally neither here nor there. That said, Def Smoke aids very nicely in chain kills if Dieck runs Galeforce, and his native Threat. Atk/Def is sufficient if you can bait incoming enemies safely.

Weapon: Tiger-Roar Axe

Assist: Positional Assist

Special: Noontime / Sol / Bonfire

Passive A: Distant Counter

Passive B: Null Follow-Up / Spurn / Repel / Close Call

Passive C: Pulse Smoke / Atk Smoke / Joint Drive Spd / Joint Drive Atk

Sacred Seal: Spd/Res Solo / Kestrel Stance / Atk/Spd Bond

• Thanks to Dieck's solid statline across all categories as well as his unique weapon by its lonesome, the Wounded Tiger is more than capable of functioning as an all-purpose omnitank, especially for Aether Raids. Due to Dieck's unrefined base 40 HP and 25 Res, however, a bit of discretion is advised as repeated attacks—especially magical—may bring him down quickly without proper setup or support.
• Tiger-Roar Axe already gives Dieck a substantial amount of additional offensive and defensive strengths.
  • It permanently reduces cooldown on Dieck's chosen Special, meaning Noontime can activate on absolutely any counterattack (unless the foe has some form of Guard), and Sol becomes a more reliable option with Geirskögul support.
  • If Dieck is two spaces within his allies (a rather common scenario when tanking incoming attacks anyway), he is granted a nice additional spectrum of stats of +5 all-around as well as an additional follow up attack. The latter in particular is vital as it provides a form of insurance against Impact skills.
• Distant Counter, of course, is a formality with omnitanks as it significantly increases the scope of targets Dieck can counterattack, especially those mages he's flipping the chessboard on.
• Null Follow-Up is a default skill that works beautifully on this build; it shuts down other units' attempts at using their own artificial follow-up skills (such as Edelgard's Black Eagle Rule or Bramimond's Void Tome) while also allowing Dieck to complete his own doubles against the ever-dreaded Impact skill line.
  • Given that Tiger-Roar Axe already provides a less stable form of that latter effect (provided Dieck naturally wins the Speed check), however, it may be worth considering a damage reduction skill instead. Spurn, Close Call, or Repel all actively reduce incoming damage depending on how much Speed Dieck has over his enemy, resulting in generally improved survivability compared to having Null-Follow Up.
• Spd/Res Solo is this build's preferred Sacred Seal for most intents and purposes, as it increases two forms of bulk at once; Dieck is always welcoming of more raw Resistance, and having more Speed has its perks in natural follow-ups (should Tiger-Roar Axe's effect fail for some reason) while factoring in Spurn/Close Call/Repel damage reduction. Kestrel Stance is a solid alternative that offers slightly less Speed but increases Dieck's damage output in return. Offshooting this, Atk/Spd Bond synergizes with Tiger-Roar Axe's preference to ally nearness, but is not recommended in Aether Raids as it common skills in the mode tend to punish adjacency.
• A smoke skill as always is helpful in debilitating incoming enemies if Dieck must face multiple rounds of combat in one sitting. Atk Smoke is easiest to recommend and access for direct damage, whereas Pulse Smoke is connivingly useful against enemy setups that rely on precharged Specials. If you have Joint Drive skills, those work too as they synergize with Tiger-Roar Axe's nearness effects while offering consistent in-combat boosts.

• Dieck's base kit provides a nice default framework for melee tanking; the only real changes he needs to make from it is inheriting a positional Assist skill of your choice as well as a superior Special to his default Luna. Your choices for the latter include Sol or Noontime for longterm sustain and Bonfire typically more consistent burst damage.
• While Close Def 4's dual defensive boost and pseudo Lull effect are quite great already, it may not be the most desirable choice as it barely benefits Dieck's damage output (aside from increased Bonfire damage). In this case, you may consider running Fury as an alternative budget choice, or preferably Atk/Def Solo if you happen to happen to have that around. Conversely, said skill is his preferred Sacred Seal choice as it further augments his Defense as well as provide a nice offensive boost.
• There's little need to replace Null Follow-Up as it's a perfectly good choice especially against the numerous types of artificial follow-up manipulation common in recent units (such as Quick Riposte, Bold Fighter, etc).
• Keeping Threat. Atk/Def is valid to keep inheritance costs low as well as weaken incoming enemies if properly positioned. If this isn't your cup of tea, the ever-conventional Atk Smoke is also on the menu.