Here are his attacks, and how to dodge/avoid them:
Attack 1: Endure Burst
Doppelganger will stab his sword into the ground, just like a Male Swordman's Endure, and sending out an energy burst around him, dealing damage to players nearby. He only performs this attack twice, at the beginning of the battle and at half-health. To avoid it, just jump over it or stay far away from him.
Attack 2: Swordman Combo
Doppelganger stops running around the screen, taunts (much like Male Swordman's Provoke) at the players, and perform a three-times Male Swordman combo attack, a punch, a stab, and a headbutt. Each hit will deal damage to the players. To dodge this, stay far away from him.
Attack 3: Summon Nightmares
Doppelganger will taunt at the players and two Nightmares will run across the screen from behind him, running over and dealing damage to the players, sending them into the air too. To avoid this, time yourself and jump over the Nightmares when they are about to run over you.
Attack 4: Low Pierce Quake
Doppelganger will jumps high into the air. He then points his sword downwards, and stabs down into the ground, like a Male Swordman's Low Pierce, creating an earthquake and dealing damage to players on the ground. This attack stuns the player too. To avoid this, jump when he's about to land.
Attack 5: Dark Shockwave x2
Doppelganger jumps forwards into the air and launches an aerial dark shockwave. After he landed, he launches a ground dark shockwave. Each shockwave deals damage when it hits the players. Doppelganger can also deal damage if the players stay too close to him when he swings his sword. To dodge this, stay on the ground for the aerial shockwave, and jump over the ground shockwave before it hits. Also, stay away from the Doppelganger.
Attack 6: Heal
Attack 7 (At Half-health): Dark Magnum Break
Doppelganger stops running around and jumps backwards. He then performs a Magnum Break like the Male Swordman's, dealing damage to any players caught in it. Doppelganger will be invulnerable during the attack. As the attack ends, he launches two ground dark shockwaves from both side of him. To avoid this, stay away from him before he performs the Magnum Break, and jumps over the shockwaves before they hit.
Attack 8 (At Half-health): Sword Charge
Doppelganger stops running, taunts at the players, and performs an attack similiar to the Male Swordman's Tackle. The attack deals attack to any players he run into. To avoid this, jump across him as he closes in.
Attack 9 (At Half-health): Dark Shockwave x4
Doppelganger stops running and jumps backwards. He will then jumps forward, launching an aerial shockwave, and create a ground shockwave after he landed. He jumps backwards again, launching another aerial shockwave, and performs a Male Swordman's Bash, launching the final ground shockwave. Players staying to close to him will also be hurt by Doppelganger's sword. To dodge this, stay on the ground for aerial shockwaves, and jump for the ground shockwaves. Avoid staying too close to the Doppelganger too.