before u challenge them make sure u have lots of full restores and revives  and that your
team has full health also make sure that your pkmn are over lv 50

Loreli has ice water pokemon so use electric, grass, fire,or water
however b awere that ice pkmn may nknow water moves so fire may be bad along with grass
water will be at an equal level so its good to use ( u can also use any fire grass or
electric move known by a non electric fire or grass pkmn)

next is bruno who has fighting rock
so water and flyig are good here
water against the rock/ground and flying against the flying 

after bruno is agatha who has Posion pkmn 
however she has flying and ghost pkmn so use electric ice and dark
electric and ice for her golbat(s) and use dark for her ghost

then comes lance leader of the E4 
he has dragons so use dragon or ice moves however electric will aslo work on some of his
pkmn
dragon and ice moves work well on everyone but electric will work on Dragonite(s) and
Gyardos

then comes the Champion your rival
i suggest that you use your strongest pokemon for him

*remember to use your items after every battle