Crisis Core Final Fantasy VII: Reunion has plenty of missable Trophies/Achievements, especially throughout the main story. The Immovable Object is one such Trophy/Achievement in Chapter 6.
Defeated all enemies in Defense of the Junon Perimeter.
How to Defeat all the Enemies in the Defense of the Junon Perimeter
Chapter 6 in Crisis Core Final Fantasy VII: Reunion is a short one taking place at Junon. That means there’s very little in the way of missable Trophies/Achievements and the Immovable Object is the only one of your main concerns outside of triggering the condition for Zack’s Fan Club. As you progress through the Chapter, you’ll eventually catch up to Hollander in Upper Junon who proceeds to unleash a group of mechs to stop you from following him.
Despite being billed as a mini-game, this is simply an exercise in defeating all of the enemies you see. The goal for this Trophy/Achievement is to ensure that none of the mechs reach the evacuation area, denoted with a purple line. If one of the mechs reach that line, you’ll want to reload your latest save and try again.
To begin with, make sure you run over to where that purple line is and turn around to face the incoming enemies. This will ensure that you can see them all as they appear. The goal is to defeat 30 of these enemies, and more will spawn as you come into contact with one of the mechs on the screen. These all count toward the 30 total you need to stop the attack.
As for the enemies themselves, there’s nothing new here you haven’t already dealt with. As all of the enemies are mechs, you’ll find that all of them, like the enemies, are weak to so having materia such as , (you may not have these if you’ve not been completing a lot of Missions), and other high-level Thunder-based Materia at your disposal will make life easier.
It also bears mentioning that the waves of enemies will stop while you’re currently in battle, so you won’t need to worry about trying to defeat the enemies as quickly as possible. Outside of this, there’s not much more to mention about this mini-game, it’s a simple one and so long as you have Zack near the purple line, you’ll be able to make sure you don’t miss any mechs coming your way.
Once you defeat the 30th and final mech, the mini-game will come to an end and you’ll unlock the Immovable Object Trophy/Achievement.