The cast time is not Instant (could you imagine if it was?) It takes approximately 11-12 seconds to complete the cast. Figured it out so I could stick /wait 12 /equip main "Earth staff" into my macro afterwards, helps with not taking as much dmg if you pull hate from the cast.
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Server: Odin Jobs: Black Mage: 75, White Mage: 37, Red Mage: 37 Merit Points (174) Levels: Attributes/Other: MP 8, Int 5, Emnity -4, Spell Interruption Rate -8%, Evasion +8 Magic Skills: Elemental 7, Enfeebling 1 Potency: Lightning 5, Ice 5 Ancient Magic II: Freeze II (level 3), Burst II, Tornado II, Flood II Genie/Novio: O Crafts: Cooking: 96, Alchemy: 51
Yup, this is definately the most damaging spell we get. Makes a very cool sound that almost makes you feel like shivering. Makes you feel like someone is scraping a chalkboard. Insane damage for sure.
Personal best of 1859 DMG on a magic burst in Ro'Maeve on Darksteel Golem. Dual thunder weather on darksday with thunder staff. Should be over 2000 if on lightningsday w/ dual weather, and all 3 added effects kick in.