I have counter barrier, boost CP, and some random thing like pdef or mdef from necro... don't even remember. I made all my subs before subclass skills came out so I'm stuck with necro don't want to xp another one :)
The BEST combo you can have for all around gameplay would be counter barrier from enchanter, boost CP from warrior, and counter boost pdef/mdef from knight. Boost CP is nice, not the best one, but 800 CP on a nuker is pretty good. Counter barrier is also good for solo pvp and oly, but if you have a healer with you it can actually get annoying. Counter pdef/mdef is good for just about anything, the only problem you might have with it is it takes a buff slot but soon that won't happen anymore!
1) Without counter celestial you are nothing, so enchanter sub is obliged.
2) Second good trigger is from tank subclass (UD +20% m.def, p.def).
3) Boost CP/Greater m. def or something what you actually need.
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I thought about summoner trigger too, but isn´t so well as I expected. Yes I know that +10% m.atack and casting speed is good, but this trigger don´t work as normal ones (when you are hitting). You must be hitted to trigger this one sub skill, so almost useless for nuker.
1) Without counter celestial you are nothing, so enchanter sub is obliged.
2) Second good trigger is from tank subclass (UD +20% m.def, p.def).
3) Boost CP/Greater m. def or something what you actually need.
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I thought about summoner trigger too, but isn´t so well as I expected. Yes I know that +10% m.atack and casting speed is good, but this trigger don´t work as normal ones (when you are hitting). You must be hitted to trigger this one sub skill, so almost useless for nuker.
Number 1 and 2 also require getting hit to activate, but you deemed them usefull.