Miss Fortune vs Vayne

Fire Double Up (Q) at a dying minion in front of a champion: if the shot kills the minion, the bounce onto the champion always crits.

Switch targets often: Love Tap (passive) adds damage on each new target, and lowers the cooldown of Strut (W).

Activate Strut (W) to get in position before Bullet Time (R): you instantly gain its max move speed bonus, plus attack speed.

Cast Bullet Time (R) on enemies who are CC'd or bunched up in a chokepoint: it's a channeled cone, and each wave can crit.

Don't stand with your back to a wall against her: Condemn (E) knocks you back, and if you hit terrain, you're stunned.

Take short, repeated trades: she's fragile, and on the third consecutive hit on you, Silver Bolts (W) adds true damage based on your max health.

During Final Hour (R), back off rather than risk your life: each champion she kills after damaging them extends the effect.