What's the best way to play Syous? He has awesome spells, but they're pretty much only enough to wipe out one screen of enemies, not to mention that he gets on other players' nerves pausing the action all the time. Should I go with using his wands all the time instead?
Yep, pausing the action every now and then can get on other players' nerves. To get around that and make them actually appreciate it, time your spells so that they save them (ex. when the players themselves are getting hit themselves, and especially to provide invulnerability through stuff like the red dragons' rock shower). Alternatively, use them all at once when the boss is out of range. The most "unnerving" component of his spells for other players is being interrupted in the middle of a combo, so as long as you don't do that often, they shouldn't mind (if they do, it's them who should adapt!).
Aside from that, Syous (and D'Raven) have that nasty trick dagger of them (forward + attack). It has a 5% chance of delivering a critical hit that takes a huge chunk out of bosses bars (I believe it's actually a percentage of the life of each enemy it hits, so it's more effective on bosses). To maximize dagger strikes, chain them along with normal attacks and "hold attack". The Magic Users can combo all these exceptionally well. Finally, his "from above" attack on downed enemies seems to deal a lot more damage than his normal attacks, since he also uses his dagger.
Other than that, in multiplayer games nobody should complain if you stay away from normal enemies; you're the least durable party member after all, and the boss killer. Indeed, they should actually go out of their way to protect you.