Remember Alder back at Floccesy Town ? The former Pokemon Champion gave you some advice near the beginning of the game and had you battle against his students. Now that you've become Champion yourself, you can challenge Alder back at his house in Floccesy.
Trainer Battle: Pokemon Trainer Alder
Note: Volcarona is holding a Sitrus Berry
As the former Champ, Alder sure doesn't pull any punches. His Pokemon are all higher Level than Iris's Pokemon, but only slightly so you should be able to manage.
First out the gate is Accelgor , who's super speedy and hits hard, but has fragile defences. Avoid Rock-types as Accelgor can batter them with Giga Drain or Focus Blast. Flying- and Fire-types should be your go-to Pokemon and ought to prevent Accelgor from knocking the wind out of you.
Next up can be Accelgor's evolution partner, Escavalier . Since it's Bug/Steel, it doesn't have many weaknesses. However Fire-type moves will absolutely tear through it. Neutral types are generally okay, but stay clear of Rock-types again because of its Steel- and Fighting- moves.
Once the bugs are down, you've got to tame a rampaging Bouffalant . Fighting-type Pokemon will perform well here. Just watch out for its highly damaging Head Smash and Wild Charge moves. As these moves do recoil damage, you can use Revives and Potions to outlast it.
You're halfway there! Next to be your opponent can be Conkeldurr . Unless they're really strong, Flying-type Pokemon should sit it out because of Rock Slide. Psychic-type Pokemon will have less to worry about. Remember not to inflict it with an ailment in case Guts activates.
Braviary should be tackled with Electric-type Pokemon. Other types will probably be afraid of its sky-high Attack and powerful moves including Superpower and Aerial Ace. If it's any consolation, Superpower will lower Braviary's Attack and Defence, making it gradually less threatening.
Finally we come to Volcarona , the epic Sun Pokemon. Quickly pummel it with Rock-type Pokemon or moves for 4x damage or Water-, Flying- ones for 2x damage before it gets a chance to use Quiver Dance. Its Special Attack is already through the roof and you don't want it getting any higher.
After what was probably a gruelling fight, Alder's grandson Benga will rush in. You'll be informed of some exciting happenings at Black City or White Forest and then he'll leave as quickly as he came in. Now you can participate in the Unova Challenge in either location.