May I add that you can use Phantom Force to maximize damage from Leech Seed, W-o-W and have more turns of harvest, enabling it to stall effectively. Paired with a great attack stat, Trevenant can do a lot of damage regardless.
Medicham for A Rank
+Obtainable after Gym 1 (in the cave before Sheridan)
+Learns High Jump Kick relatively early (level 28 as a Meditite)
+Good way to kill Audinos - High Jump Kick generally one shots them
+Pure Power enables it to 2-3HKO the v7 Kyogre, depending on EVs, IV, Nature etc.
+Is generally able to kill at least one thing per gym/boss battle
+Good coverage with elemental punches
+Detect for stalling/taking attacks in doubles
-Evolves somewhat late - requires to go over the level for Marianette
-Speed isn't too impressive
-Frail - Medicham is a glass cannon and generally can't take supereffective hits well.
-Physical Phychic STAB and Elemental Punches are only available through relearning