How do you turn a skill chain into a magic burst and how do youi know what spell to use and when to use it?
The trick is to cast a magic spell of the appropriate element within a window of 4 seconds (or five?) of a Skill Chain being completed.
A quick rule of thumb: When you see the message "<player> readies <weapon skill>" for the last person in the skill chain, you want to start casting the appropriate magic spell. You'll ususally finish up the spell a second or three behind the weapons skill finishing.
The skill chain names only sometimes give you a bump of direction as to which element you want to use. Here's a quick cheat sheet I use:
Level 1 Skill Chains -> Elements
Reverberation -> Water
Compression -> Dark
Transfixion -> Light
Detonation -> Wind
Liquefaction -> Fire
Impaction -> Thunder
Induration -> Ice
Scission -> Earth
Level 2 Skill Chains -> Elements
Gravitation -> Dark or Earth
Fusion -> Fire or Light
Distortion -> Water or Ice
Fragmentation -> Thunder or Wind
And keep in mind, you don't have to just Magic Burst nukes, and also magic burst efeebles like Paralyze (Ice based), Rasp (Earth), etc.