Detectable at login. And no , it can't be fixed unless you can reverse-engineer the damn program .
I won't post how because I would find it on 341231 l2j servers in a few days if I did.
Be smart , use l2.net
a buffed level 85 archer will have 500 crit rate. I've seen archers criting 10-20 times in a row so how do you know it's normal hit?
at 4k patk (not unusual) on a 850 p def (dc robe , not full buffed) he will hit you for 4.000x71x1.05/700 . That means 500 damage on normal hit. 500 damage ~= 4-7k crit
Of course it might be that your server's damage formulas are screwed in which case...
I usually play on 10-20x , B grade max in shops , no custom things , no tatoo.
Less than 10x it takes too much , though I usually have the upper hand cause I bot a lot. Over 20x is not fun anymore :)
Baium+Aq 60% critical damage
Zealot 300%
Hawk Totem another 30% +
Cov , Dance of Fire , DW ->200%
Bow critical range = 3x
Blessing of queen another 20-30%
That means a 4k critical on a mage , in DC robe without +++ is not impossible. I normally crit mages for 2-3k with fist weapons so...