Get a player with high hitpoints to absorb the melee, and get as many shooters (paladins or mages) as possible attacking the hydra from a diagonal direction. Hydras give great exp with a comparatively low risk of death. More attackers will help counteract the hydra's frequent healing and reduce the ammunition/rune cost of killing it.
Also a team may consist of 2 mages, one preferably a druid sioing the knight blocking to reduce health potion consumption, and both summon 2 Demon Skeletons each and HMM, while the knight stands diagonal of the Hydra while the Summons attack. Also hunted alone by high level knights, at level 110 or more.
A team of 60 level knight and two level 40-50 mages works very good but the mages should use magic shield all the time because the hydras can do pretty strong attacks.
At level 80 with assassin stars and 80+ distance a royal paladin can easily take hydras alone, but the spawn needs to be an open area and it will be a waste.
What works the best is a team of 2 royal paladins using enchanted spears along with Ethereal Spear spell runing while hitting the hydra, it is also a lot helpful to have 85+ your distance skill. It is the best way to get some profit.