Someone said before it takes top 50 ranked battle rank.
That person didn't have enough bp to be in top 50 power ranking, but was in top 50 ranked ranking, and it did let him plunder. hope it helps.:)
That would be me! I definitely can do Plunder being in Top 50 Ranked but out of the Top 50 Overall, while a group mate is in Top 50 Overall but out of Top 50 Ranked, and he can't do Plunder.