Finally, six months after beating the league (see post above), I have cleared Stark Mountain in Pokemon Platinum.
My team has remained pretty much the same, with only some levels gained and some new moves learned:
[spoiler] CURRENT TEAM
Staraptor, Lv. 53
Brave Bird, Close Combat, Quick Attack, Aerial Ace
Intimidate She is my actual starter and, while I haven't used her this much in the late game, is still able of doing very high damage, and has always been useful. Quick Attack is there because it is a priority move and Aerial Ace is there because of Aaron's Yanmega (who spams Double Team all the time).
Crobat, Lv. 52
Wing Attack, Fly, Toxic, Bite
Inner Focus The second member of the team, and revenge killer of choice thanks to the Jolly nature, but can also "stall" using Toxic and Fly - At first I chose Crobat as the user of the HM move, but then I realized that Fly was actually a very good move on it. Bite was used to counter Psychic types during the first half of the run, but I kept that move until the end despite the low base power.
Togekiss, Lv. 52
Wish, Flamethrower, Aura Sphere, Air Slash
Hustle I actually was slightly disappointed for the lack of Serene Grace and the late evolution into Togetic (Golbat evolved sooner), but his movepool and defensive stats make up for this. Togekiss is my primary special attacker, my answer to Bronzong (a lot of people use it), and also a good defensive Pokemon.
Gliscor, Lv. 53
Swords Dance, Earthquake, U-Turn, Thunder Fang
Sand Veil Like with Togekiss, I'd have preferred Hyper Cutter as my team is generally hindered by the Sandstorm, but it's no big deal. Gliscor has great physical defense, a nice attack stat that can be boosted with Swords Dance, and can counter two of the weaknesses of the Flying type. Thunder Fang is here to hit pokemon such as Gyarados, or other water types.
Gyarados, Lv. 53
Dragon Dance, Waterfall, Surf, Ice Fang
Intimidate Usually I avoid using Gyarados in most runs: Magikarp is completely useless until it evolves, and even after the evolution, Gyarados doesn't know any good move. But, considering that Gyarados' alternatives weren't this good either (Pelipper and Mantine, which are obtained only later in the game and are just defensive Pokemon), I decided to train up Magikarp... and, now I see why it is axed in Reborn, despite being so hard or frustrating to train. Gyarados was bulky enough to set up with dragon Dance on Cynthia's Spiritomb and then KO five of her Pokemon, and has also been really useful in other battles. She is my general answer to Ice types, as she is the only member of my team that is not weak to Ice-type moves.
Yanmega, Lv. 50
Ancient Power, Silver Wind, Giga Drain, Detect
Speed Boost Yanmega was supposed to be a good special attacker and an answer to Psychic types, but learns most of his good moves late (like Bug Buzz), this way being stuck with moves with low base power even after the League. This made him become my choice for a Pokemon to box, in order to place an HM slave on my team (my "starter" Chimchar), for both Victory Road and Stark Mountain. But, although he is the least strong member of my team, it is because he hasn't reached its full potential... just wait some more levels.
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Beautifly, Lv. 23
Morning Sun, Stun Spore, Gust, Absorb
Swarm Thanks to Absorb, Beautifly defeated, all by herself, Roark's gym (so it is because of her that I finished the run). I just boxed her because she was going to be replaced by Yanmega.
Tropius, Lv. 31
Razor Leaf, Stomp, Fly, Magic Leaf
Solar Power I tried to use Tropius against Crasher Wake, and she helped taking out Quagsire, but then I boxed her to get Gyarados (previously boxed), seeing that Yanmega was already supposed to have Giga Drain.
Chimchar, Lv. 5
Cut, Strength, Rock Climb, Rock Smash
Blaze My starter actually became the HM slave, sharing this infamous role with a Bidoof and a Raticate, but actually being the best of the three at it... Although he didn't battle, he still needs some recognition because he allowed me to overcome great obstacles like small trees, boulders, rocks and rocky walls, allowing me to reach the Pokemon League, and the room where the Magma Stone is, inside of Stark Mountain.
[/spoiler]
...as you can see, I don't like grinding, and I always try to avoid it. I have always seen people (mostly in playthroughs) suggesting to grind a lot before Platinum's league challenge, or going in with level 60 Pokemon to match Cynthia's levels, and I also did this in my first run; so beating the league, and then clearing Stark Mountain, with a team of six underleveled Flying-types, (with the levels matching the wild Pokemon of the area) was really rewarding - of course, it was also much more challenging and fun than usual!
I could probably do another update when/if I'll beat the League again, catch legendaries, but I'll actually be mostly in the Battle house or in the Battle Frontier from now on :)