Instant cast can be achieved in two ways.
1. A base total of 150 dex
Linkers have a job bonus of 12 dex at job level 50 so- 12 dex
99 possible status points put into dex- 99
Kahos- 20 dex
Monsoon garment- 5 dex
Moonlight/Gargoyle shoes- 5 dex
Rocker card- 1 dex
Quad dex rod/ dex survivor rod- 4 dex
x2 sting gloves/celebs- 10 dex
Total- 156 dex
So your free to put less points into dex/or switch a few pieces of equipment off.
(Note that sting brys give +9 dex for each one, so its possible to get 164 dex at most without using status foods/concentration box/other spell buffs)
Status foods give 1-10 dex and last for 30 minutes, they disappear upon death
Box of concentration gives +3 dex and lasts for one minute
Possible player buffs that increase dex include but are not limited to blessing- +10 dex for a short period of time. The guardian angel for extended classes also grant a level 10 blessing upon leveling up.
2. Cast reductions
x2 berze accessories
x1 dark illusion
Song of Bragi- Bard skill
With all three, you can hit ic.
As a linker though, you'll do fine without instant cast.
For slow moving monsters like stings; use estin for pushback then esma. Rinse and repeat.
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As Lost said, +4 more dex helps. But ic is an expensive thing to get for those new to the game.