Your mileage may vary with Platinum - some people find the Pokemon League quite challenging, some find it fairly easy. I beat it with a team of 6 at Level 50 (Empoleon, Lucario, Luxray, Glaceon, Togekiss and Garchomp) and didn't have major issues, but if you're struggling now before this point you may want to first get your levels up to scratch. Level 55 should be fine for taking on the Elite 4 - rematches using the Vs. Seeker, taking on the Elite 4 early and using their levels/Pokemon for experience before losing or spamming wild Pokemon in Victory Road should make that a fairly smooth (albeit boring) process.
You'll probably want a sixth Pokemon, Altaria honestly isn't that bad a Pokemon so if you wanted to keep it because you like it I definitely would! Otherwise I'd suggest adding Luxray to your squad if only for Cynthia's Togekiss. Luxio can be caught on the route before Sunyshore City in the early 40s I believe? A moveset of Thunderbolt, Crunch and two other moves should suffice (Charge + Strength are decent options, although you may need a Heart Scale for Charge). Try getting one with Intimidate! If you're set on replacing Altaria I think Staraptor is the best choice here - Close Combat + Fly + Normal STAB (Return with high enough friendship, or Strength) is all you really need.
Anyway, here's my moveset changes/recommendations!
Empoleon:
- Mostly fine
- Delete Rock Smash in Canalave City and replace with Flash Cannon or Grass Knot
Roserade:
- Also mostly fine
- Replace Toxic Spikes with Shadow Ball (if possible)
Weavile:
- Cut needs to go, as does Focus Punch - the former is super weak and the latter requires you to not be hit whilst focusing, which is never going to happen
- Teach it Night Slash (Heart Scale required, Serebii lists locations to find them without requiring the Underground:
https://www.serebii.net/itemdex/heartscale.shtml)
- Teach it Ice Punch (requires 6 Blue Shards and 2 Red Shards - these should all be available before the post-game? You can trade Star Pieces for shards at the Fuego Ironworks if you have any to hand)
Altaria:
- Is fine, no issues here really
- Consider replacing Sing with Earthquake or Flamethrower? It's not necessary though!
Gallade:
- Needs Psycho Cut (Heart Scale required, see above)
- If you have spare Shards you could give it an elemental punch? Ice Punch would be my choice! Could replace Swords Dance then
- Close Combat over Brick Break once you reach that level
I'll even throw in some advice for the Elite 4:
- Buy plenty of Hyper Potions and Revives! Full Restores or Full Heals are also advised
- Save before every fight, let's you test what is and isn't possible
- Aaron is pretty easy - Empoleon can beat everything aside from Heracross and Drapion? Altaria handles Heracross (be careful of Stone Edge) whilst Gallade can likely 2HKO Drapion with Close Combat (if not, the rest of your team handles it)
- Bertha is actually quite challenging I've found - Roserade should one-shot Whiscash and handle Hippowdon, Golem and Rhyperior pretty well (Empoleon covering where Roserade falters) with Weavile taking down Gliscor with a speedy Ice Punch
- Flint can be tough, but a combination of Altaria + Empoleon + Gallade should see you through it - try out setting up Dragon Dance with Altaria versus Houndoom and then KOing everything with Earthquake
- Lucian is a pain - Weavile handles the scary threats in Espeon and Alakazam and might even beat Mr. Mime? Bronzong will take a team effort and his Gallade might give you some trouble, but Altaria + Luxray + your own Gallade should prevail
- Cynthia is scary - you could try setting up Dragon Dance with Altaria and sweeping but I suspect her Milotic + Togekiss ruin that for you (maybe give it a go?). Spiritomb is walled by Empoleon, Roserade goes down to Altaria, Lucario falls to Gallade, Luxray would be solid for Togekiss + Milotic, Garchomp gets hopefully outsped and KOed by Weavile? Other teammates of yours can handle other threats as necessary.
Good luck!
