Pikachu: Light screen
Feint is only usable when your opponet uses detect or protect so I don't see why that's such a popular choice. It would hardly be used. Brick break would no doubt look similar to iron tail and, again, it would require the writers to spam the hell out of light screen and reflect for it's secondary effect to kick in. I really doubt they will teach Ash's pokemon attacks simply to defeat Paul. Light screen seems to defend pokemon even while they are in motion so it would be great for Pikachu. He could rush towards his opponets with volt tackle and not have to worry about being stopped by beam attacks. It would also protect the rest of Ash's pokemon so it would be a huge help.
Grotle: Sunny day+solar beam, giga drain, earth power
If you wanna add a bit of strategy, then make Grotle a sunny beamer. Sunny day, solar beam, and synthesis along with Grotle's defense would make it a pretty formidal opponet. Solar beam is a powerful grass type attack and one of the only strong attacks that would separate Ash's Grotle from Paul's Torterra. I don't think leaf storm would be the best choice statistically. It would lower Grotle's strength and then what would it have left? Grotle's whole strategy is being able to take hits while countering with powerful hits right? It won't do Ash a bit of good if Grotle could no longer deliver powerful hits. Maybe giga drain could help Grotle heal and help with it's defensive strategy and earth power could add some type variety.
Monferno:Flare blitz, focus blast/close combat, and blast burn
No brick break, no focus punch, no fire punch. They all look too similar to mach punch. Flare blitz could be a powerhouse physical move for Monferno since it seems to be pretty agile. Focus blast would be a better pick for a fighting attack than any of the other ones in Monferno/Infernape's movepool. Staraptor already has close combat but the monkey may learn it too rather than learning focus blast. Lucario's close combat was completely different from Staraptor's so who knows. Blast burn is a really strong attack that it will definately learn if it reaches it's final stage.
Buizel: Razor wind/waterfall, ice punch/fang/wind, rain dance
Razor wind is the strongest attack in it's level up move pool so it can add some power to Buizel's move set or it can learn waterfall instead but I really don't know how they would portray that in the anime. Buizel can learn ice punch or icy wind so that it doesn't have to rely on a stray ice attack to use ice aqua jet. Ice fang if it evolves into Floatzel. And ice attack would also add some type variety, Ash doesn't have any ice attacks on his Sinnoh team. Rain dance would turn Buizel/Floatzel into a real powerhouse. It would increase the power of it's water attacks as well as it's speed.
Staraptor: Roost, air cutter/ominous wind/twister
Roost would be a nice change of pace for Ash, another defensive attack. Ash is really going to need to go all out with attacks like roost if he ever wants to defeat Paul or win a league, more strategy is required. All of Staraptor's current damaging attacks are physical, making that Staraptor has to get close to it's opponet which sometimes causes it to get hit by special attacks. A special attack like air cutter, ominous wind, or twister would help. Twister and ominous wind would add some type variety too.
Gliscor: Earthquake/stone edge, night slash, and/or poison jab,
Earthquake and stone edge would both give Gliscor a ranged attack and a super powerful attack. Night slash and poison jab would add some nice type coverage to Ash's team.