Everyone giving hell to Serpent for him not having a Reaper need to realize his commentary on how the job plays is spot on. If you won't take it from him, take it from someone that does play Reaper.
I play Paladin extensively in PvP and I can tell you from experience that playing a Reaper is a somewhat similar experience. A Reaper is a team fighter, you want to scrap with your team using your support ability Arcane Crest. Not only do you get a damage boost, but it heals you and your team when it breaks.
Your job as a Reaper is to pick a target of opportunity and make their life absolute hell. Death Warrant alongside your front loaded burst makes you a good assassin, which actually lines up with Reaper job lore. Get in, cut the head off the snake, and get out.
Staying alive and fighting with your team also ties into you getting Plentiful Harvest stacks. A full 8 stack Harvest can be devastating in a team fight as it's not something they will see coming like an LB.
Speaking of LB, its not fire and forget like every other melee. Using it in a bad situation will result in nothing more than a wasted LB. It breaks guard and forces movement, so using it during a scrap to open the enemy up, or during environmental traps, can swing an entire fight in your teams favor.
Your survival is your own responsibility, and as a Reaper, you have the tools necessary to keep yourself alive in extended fights, allowing you to make full use of your kit.