Here's what I suggest. Choose Tepig for yourself and Axew as your sidekick. My Tepig's moveset was Tackle, Flamethrower/Ember , Assurance, and Flame Charge. My Axew had Slash, Dragon Claw, Assurance, and Scary Face. I remember trying some orbs like the One-Shot Orb and they failed, so avoid that.
If you want offensive held items, try Scope Lens or Choice Ribbon
If you want defensive held items, try Defense Band, Zinc Band, or Lucky Ribbon (if you have it)
At the start of the battle, throw a Sleep/Totter Seed and then approach. Have Axew alternate between Scary Face and Dragon Claw. If Kyreum is slowed, Dragon Claw. If he's back to normal speed, Scary Face. You should alternate between Flame Charge and Flamethrower/Ember.
If you know he's below half HP, you can switch Dragon Claw/Flamethrower to Assurance if you choose.
The goal of this strategy is to put the mechanic of speed in your favor, letting your opponent make as few moves as possible. Stuff your bag with Reviver Seeds and good luck!