fml.js

;(function batchDeleteMyMessages (channel, limit, before = undefined, loops = 1) {
  if (limit <= 0 || loops > 10) return console.log(limit, loops) // If there are no more messages or we've been here too long, leave

  // Get 100 messages from the channel
  channel.getMessages(100, before).then(messages => {
    // Get a list of all the messages from the bot
    let toDelete = messages.filter(m => m.author.id === (channel.guild.shard.client || channel._client).user.id)
    // Also save the ID of the first message so we can give to the next iteration
    const firstMessageId = messages[0].id

    // If we have more messages from the bot than we need, only take the last ones
    if (toDelete.length > limit) {
      toDelete = toDelete.slice(0 - limit)
    }

    // Delete all the messages we need to and wait until it's done
    Promise.all(toDelete.map(m => m.delete())).then(() => {
      // We deleted all those messages, now we call this function again to
      // process the next batch. We subtract the messages that we deleted here
      // from the limit for the next batch, and we provide the first message
      // in this batch so the next one knows how far back in the log to start.
      // We also increment the loops argument so we can stop ourselves from
      // making too many recursive calls to the API.
      batchDeleteMyMessages(channel, limit - toDelete.length, firstMessageId, loops + 1)
    }, err => {
      // oh god fucking damn it
      console.log(err)
    })
  })
})(channel, 50)
// ^ The arguments we pass to the function to start with: The channel that we're
//   looking through, and the number of total messages to delete.