User blog comment:B214/Volume 17 spoilers./@comment-24053170-20140220004506/@comment-1359322-20140220024859

Use the Dimensional Gap. Just create a huge training space there and have Issei battle them.

@JJJ, that's why i include mentioning Saji.

Well at least it's better than my original plan, that is to have Issei and Vali battle the 6 Dragon Kings, all the Dragons under Tannin and some caught Dragon.