Prepare to read one of the greatest troll sets of all time.
Slowbro @ Leppa Berry
Ability: Own Tempo
EVs: 252 HP / 108 DEF / 152 SDEF
Nature: Sassy (+ Special Defense, - Speed)
Slack Off: Heals Slowbro. Spam this.
Recycle: Gives Slowbro a new Leppa Berry.
Yawn: Annoys the enemy by forcing them to either switch or deal with sleep.
Disable: Shuts down powerful enemy moves.
Honestly your last two moves can be anything you want. Rest + Snore, Substitute, defensive buffs, whatever you want as long as it makes it harder for the enemy to kill Slowbro.
Strategy: Spam Slack Off over and over. Once it runs out of power points, the Leppa Berry will kick in. At some time after this, use Recycle to get a new berry, then return to spamming Slack Off. Use Yawn and Disable as needed to annoy the enemy. When Recycle itself runs out of power points, the berry will kick in and restore it as well. Continue spamming Slack Off for about 50 turns, at which point the opponent will rage quit, being unable to kill your Slowbro.
With this move set and the Leppa Berry, you can spam indefinitely barring Slowbro's tragic demise at the hands of some OP legendary or some weird move combo that would prevent healing or Recycle.