How do you remove boulders?

You use the HM called Strength

you go talk to Athena and she will tell you to talk to Hercules again then he will help with a lot

If the boulders are smooth then you need to get one of your Pokemon to learn strength to move it and if the boulder is rough then you need to get your Pokemon to learn rock smash ( both of these...

steal other players flags, when you get enough you can remove boulders

You have to meet other people and take their flags in the Underground.