Sidney:
Sidney wasn't terribly hard, and by that I mean he's the easiest of them all to beat because he lacks defense. Machop only really fought Mightyena, and despite its Intimidate, one-shotted it with Cross-Chop. Then came Shiftry, which fell easily to Koffing's Sludge Bomb. When Cacturne came along, Koffing wasn't fast or healthy enough to stand up to it, so I brought in Snorunt, who struggled to take Cacturne's hits, but did the job. Sharpedo, honestly, I forgot how I beat it, but I think it was either Shroomish or Machop. Absol, perhaps the most frail of them all, went down to a barrage of Rock Slides and Strengths from Nosepass.
Phoebe:
Phoebe took a couple tries, not because of difficulty, but because I was just being stingy. Dusclops had a high defense, so I needed something super-effective to take it out, so it was either Snorunt or Flygon. Also, since there were two, I would have to use one for each. Since the second Dusclops had Ice Beam, I decided to open with Flygon and then run into the second Dusclops with Snorunt. Eventually, I had to take the second one down with Leech Seed, and the other pokemon fell quickly to Flygon and Nosepass.
Glacia:
Glacia wasn't as hard as Phoebe. Her Glalies both fell easily to Machop's Cross Chop and Low Kick, and Sealeo to Shroomish's Leech Seed-Stun Spore combo. Her only real problem was her Walrein, which only required a few Low Kicks from Machop to go down anyway.
Drake:
Drake wasn't too difficult either, except for maybe his Salamence. Flygon did most of the work here, taking out Shelgon and both of his own Flygons, and eventually Salamence, when no one else could take him out. Altaria, however, was Snorunt's domain, until it's Take Down took them both out.
Steven, the Hoenn Region Champion:
Steven was fairly difficult, probably the hardest of the five. Skarmory was able to get in Spikes on us, but it couldn't overpower Koffing's Thunderbolt for too long. Claydol fell prey instantly to Shroomish's Leech-paralyze combo, and then couldn't pull off enough moves to keep up with the health I was sucking from him. Cradily got some critical hits in, taking out both Snorunt and Machop in one hit a piece. Ultimately, I had to use Flygon's Earthquakes to take it out. Aggron fell to a one-shot Earthquake. Metagross, Steven's star pokemon, gave me an enormous amount of trouble. I was able to take it out with two of Flygon's Earthquakes, but they were difficult to pull off when Metagross has enough power to one-shot all of my pokemon. Last off, Armaldo. Armaldo was powerful, but Flygon could easily take 3 or 4 hits before needing to heal. I wanted to try a Leech-paralyze combo on Armaldo, but Effect Spore poisoned it before I could paralyze. Eventually, it was once again Flygon, the main power of my team, that took down Armaldo, and allowed me to defeat Steven, the final trainer of Hoenn.