Invest 220 in Speed and keep the Adamant Nature. This will beat the most common variant of Choice Scarf lead Venusaur in UU (If this is for UU... If it's for NU, I can't help you much =/) who only runs 216 Speed EVs with a Modest Nature. You are both 80 Base Speed Pokémon, so you shouldn't have to invest any more than that unless you don't have a 31 Speed IV.
This way, you won't be OHKOed by Leaf Storm or be Sleep Powdered while you will have an early 6-5 lead as long as Zen Headbutt hits.
If this is for UU, Pokémon such as Nasty Plot Houndoom and Substitute Rhyperior make great partners for Medicham due to their ability to set up on Medicham's primary counter, Spiritomb, who commonly runs a WoW or Trick / Sucker Punch / Shadow Sneak / Pursuit with Leftovers or a Choice Band type of set, both of which Houndoom and Ryhperior rape. Just make sure to get Rhyperior in on anything not named Will-O-Wisp