if GS = Ghost Sentinel..
for summoner:
a) buff with focus, btb.
b) UE, evade shot to avoid pet attacks.
c) kite the horse,cat w/e - don't bother slowing it, the summoner can cure it.
d) try to land hex and pray that stun lands and that you crit on normal atkz.
It's very hard to win a summoner, they are pretty strong and if well played, it is indeed nearly impossible, especially against the summoners that have warrior bane..
However, if you play on a server with augmentation and you're having major problems only against summoners, try to get an active skill such as betray or erase... it might take a few ls, but it will do _wonders_.
for necro:
.. it's the same thing. however, if the stun lands on the necro, I'd suggest bringing the summon down while the stun effect lasts. /repeat
oh and buff with mentalshield because of doom.. ._.
Again, you can go for a darkness resistance passive augmentation.
(or active skill [unholy resistance, divine protection, w/e])
(or mdef passive, should work decently as well.)
Dynasty armors most of them also have darkness resistance.
Baylor or Frintezza unique jewels will do wonders with their +15% darkness resist.
Also, Antharas earring will work good to get you safe from Anchor. (due to its resist earth attacks +15%, helps)
Anyways, in l2 olympiad, some classes will just get their ass pwned, even if the user is skilled.
that's just the way this game works, unfortunately.
...and ghost sentinel.. lol ;/..