The Gamma spell deals an average of 5000 damage per cast, and its Crisis attack can deal over 10000 damage to the entire enemy party easily. However, even with max MP, it'll drain a third of your MP per cast, and Nuke will consume all of it. On top of that, equipping this spellcard decreases your max HP, strength, and vitality by a large amount, and its damage is completely random, scaling from one. Becomes Awesome Yet Practical when coupled with the Lord-Sorcerer's Gown (which halves mana costs) and Trick spellcard (which greatly increases all stats, undoing the stat malus of the Gamma spellcard), however...
While there is no damage cap, there are only few attacks that can ever get into five-digit damage: The very random Gamma spell in good cases, its crisis cast Nuke, Vortex (although dealing that much damage with Vortex requires really epic INT, and the damage is split into 15 random-target attacks), its crisis cast Armageddon, and possibly the Bolster crisis cast "Limit Break", though it "only" has a confirmed damage of about 7000. And, finally, Brutal Attack when backed by a sufficiently inflated strength stat. Additionally, critical hits of normal attacks by someone with >900 strength and their ultimate weapon scored on a really low-level creature with weak defense, i. e. a kobold, can deal about 10000 damage too. It's questionable though whether this counts, since a battle between a Level 5 monster and Level 99 characters is no normal situation.
The Defend Command doesn't have an effect before the defending character can actually act, making it nigh-useless against fast enemies (and making speed-increasing equipment for your MeatShield a must against the last Gate to Elysium boss).
The Crisis Cast of Bolster is a Desperation Attack called Limit Break. It deals physical damage in inverse proportion to the amount of HP left. If you only have ~5% hp left, it deals about 10 times the damage you can deal physically when you get it. Unfortunately, it requires a lot of luck, since if you try to get your char's HP sufficiently reduced, it will most likely end in them getting KOd and ruining the chance to use the spell. It's extremely powerful in the beginning, and although the damage apparently scales with your max HP, it's less powerful later because you obtain easier means of dealing similar damage.
Alexander can hardly be beaten without figuring out to give the Party Hat to your sturdiest character and keep them defending, Barriered and X-Shielded all the time, which reduces Lionheart's damage from ~2x5000 to a negligible ~2x350, and equip the Party Hat on them to make sure he won't hit anyone else.