Sure, Chaining is where it's at when you've got the right units. But for Single Unit Damage potential, units with Multi-Strike AOE Abilities are what you want. And even a single Man Killer( ) will get you to 999 easy. And 7⭐ Xande has the ability to Strike Six Times. That's 6K Damage from a single unit on the first turn unless they absorb both Dark and Earth (Dark being the least likely to be geared for). Even at 6⭐ he's able to do 4K damage alone.

As you get into the top 100 / 1000 Ranks, you'll start seeing more Elementally Immune Units (Though generally it'll just be a single annoying unit), but if you're getting that high you're not a "Newer Player", and you've probably already got a team of bruisers. ;P

Other units with High Multi-Strike Abilities on the First Turn are:

• Crimson (6K at 7⭐ and 4K at 6⭐) Non-Elemental Atk [Great Slash Combo 2x], so you can gear him TDH and hurt anything that moves with unavoidable damage. Surprisingly solid damage.

• Hyoh (4K) TDH Non-Element [Dual Slash 2x (With [Enhanced - Double Blade])]. Only 4K DW (So Dodgeable) with Unenhanced [Double Blade]. Of Extra note: He has several Multi-Hit Abilities that Hit One Enemy and then go AOE with {DR or OS Frames} & he has a chance to Counter with Ruthless Blade for Extra Damage if you go second.

• Magitek Warrior Terra (8K at 7⭐ 4K at 6⭐) Magic Non-Element (Or Fire, Wind, Light) [Chaos Abilities 2x {CWA}]. Can also do 1K Less Damage to Apply a 75% Imperil of her Elements. Excellent Damage, but Magic Falls out at Higher Levels with Ridiculously buff SPR Counter Tanks (Lilith & Lucas... -.- And annoying aRain)

• Operative Zyrus (8K Elemental 4K Non-Element at 7⭐ 2K at 6⭐) Magic (Lightning {BS}, Dark {CWA}, Ice {AZ}) [Specter Abilities 2x]. Non-Elemental with 8K on a Single Unit [Ultrasonic Weapon 1.5x].

• Adventurer Locke (6K at 7⭐ 4K at 6⭐ BUT!) TDH Non-Element {SR} Chaining [Suppression Throw 2x]. A chaining Multi-Hit TDH Non-Elemental Ability is the best you could possibly ask for in Arena. Not the most damage "Alone", but the chaining will add incredibly to whatever damage you're doing.

• Agent Olive (6K at 7⭐ 4K at 6⭐ BUT!) TDH Non-Element Chaining AND 100% Ailments [Cannon Fire - Staggering Shot - Paralyze {SR} 2x] & [Cannon Fire - Crippling Shot - Petrify {BS} 2x] with misc other Ailments each AND She's Stop Immune! Pairs Beautifully with aLocke, and is Hands Down your best Non-Limited Arena Chainer. I've learned something new in all this and I know who's going into my Arena Team now! O.o!

Dual Wield Multi-Strike (Can be Dodged, so Less Ideal)

• Pyro Glacial Lasswell (4K) DW (Ice & Wind) [Exorcising Storm 2x]. Only 2K if you want TDH Undodgeable

• Lasswell (Same as Above, but MUCH Squishier... though everyone has him).

• Dark Veritas (4k) DW Non-Element [Dual Slash 2x]. Only 2K for TDH.

• Zeno of the Beta Star (4K) DW Non-Element [Dual Slash 2x] Only 2K for TDH. He also has Multi-Hit Abilities that Hit One Enemy and then go AOE with {AMoE Frames}.

Limited:

• The Lord of Shadows (6-12K) TDH Non-Element at 6⭐ [A Cut Above 3-6x]. Ridiculously powerful unavoidable output from a 6⭐ Unit. Best in Class.

• WWFina (Or Traffic Cone Fina for those who remember): (4K) TDH Light Element [Enhanced - Divine Burst 4x] VERY Squishy, but the goal is to One Turn the Enemy anyway, so bad luck aside it rarely matters.

• Veronica (4K at 7⭐) Magic Non-Element (Or Lightning {BS}, Light {DR}) [Magic Burst 2x {AZ}]

• Xuan Wu & Qing Long (4K at 7⭐) TDH Non-Element (Magnus only 3 Casts Total) [Special Arts - Palm Strike 2x] 2K after Magnus, and Needs a SPR Heavy Build. Very weird Build needed.

• White Mage Rosa (12-16K at 7⭐ ONE UNIT) TDH Non-Element [Radiant Arrows 6-8x]. Only worth noting because this is absurd damage potential... for One Unit. Could be useful if you're dealing with Bulky Units as a "Surgical Strike" unit against those units.

• Eleven (9K at 7⭐ 6K at 6⭐ ONE UNIT) TDH Non-Element [Unbridled Blade 3x] Not quite as absurd as WM Rosa, but still a pretty big hit.

Unique

• Kaito (3K) TDH Non-Elemental [Neap Tide, High Tide, Devastating Tsunami]. Worth mentioning because as a DoT unit, Turn 1 is only 3K, but from turn 2 on it's 3-7K (4-8K at 7⭐) since he'll have both his normal attacks & the DoT effects if your Battle goes beyond 1 Turn. At which Point he can use his AOE {BS, or AZ Frames} Moves, one of which Adds Human Killer to his Attacks.

• Orlandeau's TMR (4K) DW(ish) Non-Element [Spirit Blade 2x] Only 2K TDH BUT can be equipped to any unit, which makes it particularly compelling for crappy Bonus Units. Hybrid Build needed though.

• Physalis (12K ONE UNIT) TDH Non-Elemental [Tetrashot 4x]. She's our only Non-Limited Big Single Punch unit. And she gets it at 6⭐!

• Wild Rose Firion (6K ONE UNIT) TDH Non-Elemental [Quick Arrow 3x]. Turns out we have another. Not 'quite' as hard a hitter, but worth a note.

And don't forget units with Single hit 3-4 Spells and Abilities per turn, which can fill in for any Multi-Hit units you don't have.

With the right combination of units, you can be doing an unchained undodgeable 15-22K Every Turn. Very Few units (Even among the top teams) will survive that, and it's an easy Repeat & Win, no need to time chains (Though they'll naturally chain enough to add 2-4K on top of that). These are off the top of my Head. I'll add to the list if anyone knows of others that belong here.

^{Side Note: It's some serious Bull that a phone falling asleep for too long, or reloading the app will fail arena. No Bloody Sense at all. :?}