Champion: Blue (all three Eevee variants)

Your rival reaches the title minutes before you do. His last Pokemon is an Eeveelution, and you chose which one back in Pallet Town and on Route 22.

Step 1: Know which branch you are in

Win at Oak's Lab AND win on Route 22 to face Jolteon. Win at Oak's Lab but lose or skip Route 22 to face Flareon. Lose at Oak's Lab to face Vaporeon.

Step 2: Open on Sandslash L61

Pure Ground. Water, Ice or Grass. Surf ends it.

Step 3: Remove Alakazam L59 fast

135 base Special and 120 base Speed, but 45 base Defense. Hit it with the strongest physical attacker you own before it sets up.

Step 4: Exeggutor L61

Grass/Psychic - Ice Beam or a Fire move takes it out. Beware Sleep Powder and Explosion.

Step 5: Cloyster and Ninetales / Magneton

Cloyster (Water/Ice) takes Electric. Ninetales takes Water or Rock. Magneton is Electric with 95 base Defense - hit it with Earthquake or Dig.

Step 6: Finish the Eeveelution L65

Jolteon - Ground (it has 130 base Speed, so paralysis first helps). Flareon - Water; it has 130 base Attack, so do not trade hits. Vaporeon - Electric or Grass; 130 base HP means it will not fall to one hit.

Aim for a lead around L55+ with the whole six at 48 or better, and bring Full Restores - you fought four Elite Four members to get here with no healing.