Is the mission "You Are Cordially Invited: RSVP" with Flesh-stick repeatable? I want to be able to lure him in to tiny tinas place and finish all her side missions. But If I finish the mission will he still be able to respawn or will he not respawn anymore?

no

thinking of lvling my char to 50 off him then. I'm almost lvl 41, anyone want to help me by standing next to tiny tina and restarting mission every time etc. I'll pay fg, lmk how much.

You have to kill him, it fails the mission, then you restart it. Its how you power level. Have the low level sit at Tina spam accepting the quest while the other player (the one who will do the killing (a 50)) kills Fleshstick over and over. Can do 1-50 in like 1 or 2 hours depending on if it glitches.

What platform you playing on?

Pc
I've just been soloing flesh stick over and over again lol, currently lvl 41