Despite his somewhat feminine appearance and the fact that he uses all female Pokemon,
Will is actually a guy. Yeah, just like Bugsy. These weird Johto people. Anyway, Will is
the first of the Elite Four and he uses a bunch of Psychic-type Pokemon. All of them are
equipped with the powerful Psychic-type attack, Psychic, and all of them are
part-Psychic-type. However, they each have a complementary type in addition to Psychic, so
you may be able to exploit those weaknesses instead.
The first Xatu Will sends out (he has two) likes to start off the fight with U-Turn to hit
for a fair amount of damage and then switch out for another of his Pokemon, so don't count
on actually knocking Xatu out right away unless you hit it with a strong attack. It can
also use Me First to copy your attack, strike before you, and hit you with that attack.
Jynx isn't too difficult, but Lovely Kiss can put you to sleep. Psychic is the only really
threatening move it has, as its other moves are all Physical-based and Jynx has pretty low
Attack.
Slowbro can be very annoying if you give it time to set up. It has both Curse to raise its
Attack and Defense (while lowering its Speed) and Amnesia to sharply raise its Sp. Def. It
can set those up and prevent your attacks from doing a lot of damage. Try hit it with
anything it's weak to as quickly as possible to avoid giving it much chance to set up.
Exeggutor can also put you to sleep, but Psychic is still the main pain-bringer. It's
mainly there for support, so again, try take it out right away.
Lastly there's his second Xatu, which is more attack-oriented. It has a Sitrus Berry to
heal itself if it gets weak, in addition to whatever Full Restores Will gives it.
Shadow Ball or Dark Pulse are both great moves (that are learned from TMs you should have
obtained) against all of Will's Pokemon. Otherwise his team has a pretty fair weakness to
Electric- and Ice-type attacks.
After beating Will, heal up your Pokemon with any form of Potions (start with Potions and
Super Potions and save your stronger stuff for in-battle) head into the next room to fight
the second of the Elite Four...