This is probably the easiest done in the Ladlum Caverns early on with Sonia and Kirika alone, after encountering the story boss in the area for Opus 2. Further in to the second area is a large enemy known as an Orc (also needed for a subquest from Marga). If you act as Sonia being the leader, her constant attacks with circle and triangle, then leading into Million Thrusts and repeating, while Kirika heals, you shouldn't have any real trouble reaching this.
*Addendum from Mark_Miwurdz and supported by NicholaiGinavaef:
If you've levelled up further than comfort provides and you've gotten to the point where such a target does not adequately survive for 99 hits, there is an aspect late in the game called "Only One" that you may have access to either through Rapple's quest series or as a part of clearing the story. The ingredients are a Rusty Sword from Guard Skeleton and Octopus Legs from Buoyant or Floating Octo and, once created, drops your damage down to 1 (making this far preferable after a certain level).
*Addendum from NicholaiGinavaef:
Alternatively, bold any new zone that is several levels higher than your own currently and, still using Sonia, take your challenge out that way.