Minecraft Legends

The quickest and easiest way to do this I've found is to Creeper rush a small base.

First you'll need to forge an alliance with the Creepers, once you've destroyed your first Portal base you'll be able to ally with the Zombies, Skeletons and Creepers respectively. All this will require is you defending their home from an attack and then counter-attacking and destroying a piglin base.

Once you've unlocked Creepers, find a small base that ideally is on flat land, the less time spent building bridges the better. Once you're ready, drop down a creeper spawner and spawn in 20 Creepers.

Now, with the Creepers in tow, rush the portal ignoring any enemies and structures on the way and use Banner Mode to order them all to attack the portal. Unless you've been unlucky enough to have some knocked off and/or killed before they can explode, you should destroy the portal and get your achievement.

I recommend just starting a new save on Storied difficulty for this achievement.
20 stone golems will easily take down a base as the enemies just dies by the sight of your sword at this difficulty. 😂

I recommend saving any easy bases to tack and try get firsts protectors in world to make this easier.

Secondly to leave creeper. Skellton and zombies to help free later after you upgrade yourself abit while taking out smaller bases .

Once you feel comfortable to try start one of free freindly alliances you can help and aim staright for portal and only attack that since that will destroy whole base once at 0% .

However your not invincible and your friends still need be able to access portal to attack . I would even recommend a upgrade to remove nether rack so a structure that creates small green land can be expanded so you can build little spawners for more allies to attack. This will be trial and error so do practice and get used to control's and make sure you feel prepared.