It's HR? If it's, does he have Tao? or Ghost? Full reflection equip? (i mean, Strong Shield, Alice, High Orc, Maya, Valky Manteau).
If playing Tao, first of all, divest him or break his armor (if not using Valky). Try using Wikebine Tres Card to divest him. You can also try Atroce Sword or Mailbreaker (if sloted, you can also try using Metaling Card to take his weapon off) if you know that not using Valky (besides it's almost sure wearing that). Try using Death Guideance if avaiable in your Server, that may solve your problem, regaining HP while damaging (also Sniper MvP Card will work). The way to win is having more HP that him and try to avoid all (or almost all) damage receive (or regain some HP while damaging). If Wearing Tao, you have to take it of from him, that way you reduce his Max HP to a half. You must first atack him by Bashing, Spiral, anyway. After you are almost die (use mastelas/berrys if possible to avoid this), play berserk (frenzy). This restores all your HP (if you have Tao, this must be enough), reach 190 ASPD and try killing him by DPS using Death Guidance (if possible). If not possible, you can try Execuitoner (if wearing Archangeling or Pali Combo, it will be painful for him) or any PvP carded weapon. You can also try using Ranged Skills to avoid reflection, but Defending Aura will make this hard.
@Sarin: Is true that reflecting damage will be also reflected? Cause for example, I deal damage, you reflect this damage and I also reflect damage reflected, and you reflect the reflection of damage dealt, and so on... =S Kind of cyclic damage dealing.
Saludos.-