Will-2xatu lv.40, lv.42, jynx- lv. 41, slowbro- lv.41, exeggutor-lv. 41.
Koga-Ariados lv.40, venemoth-lv. 41, forretress- lv.43
Muk- lv.42, crobat- lv.44
Bruno- Hitmontop- lv.42, Hitmonlee- lv.42, Hitmonchan, lv.42,
Onix- lv.43, machamp- lv.46.
Karen- Umbreon- lv.42, Vileplume- lv.42, gengar- 45,
murkrow- lv.44, houndoom- lv. 47.
And the leader, Lance. Gyarados- lv.44, 3Dragonite- lv. 49, 49, 50,
Aerodactyl- lv.48, charizard- lv.48.
After you have gotten all of the 16 badges, they have stronger pokemon, and they have some new ones too. I will leave that as a surprise for you, but if you want to know, leave it as its own question:)