1. Tnt cannons are great, but can mess up randomly, take a lot of redstone/tnt, and take a lot of time too. An easier and more effective way is to run around and get some creepers... :D
2. Blast proof base? See if they have a roof. If not, you can ender pearl/1x1 tower+jump your way in :D
3. Roof and blast proof base? Use x-ray or be legit if you want and look underground around their base. Chances are, they didn't claim all of their minds, meaning that you can enter their base that way.
4. No way to get in at all? Kill their members so that they lose their power and you can steal their land.
5. Impossible to kill members because they are pussies?- Spam lava around their base so it is impossible to escape, make an alternate account and get access to the faction that way, or HACK MWAHAHA

THanks for reading this tutorial, plz diamond/subscribe!

BY THE WAY:
I AM NOT ADVERTISING HACKING, BUT SIMPLY MAKING A JOKE. DO NOT HACK, IT IS MEAN AND UNFAIR. HACK IN SINGLEPLAYER IF YOU WANT TO.