Blaziken is the most recommended Pokemon for taking on the Elite Four for a good reason. The Attack, Special Attack and Speed allow Blaziken to apply to the one-hit-per-pokemon strategy when it is at least level 59. My recommendation for the moves to use are Flamethrower, Fire Blast, Sky Uppercut, and your choice of Brick Break, Double Kick, or Mirror Move. These moves allow you to have an easy time for the first, third, and fifth battle due to type advantages.