X1 is a giant Krogan boss encountered about two thirds of the way through. The achievement is story related and can’t be missed.
He has a few attacks but his main ones are a homing biotic attack and grenades. The best way I found to defeat him is to slide in circles until your cloak is charged then cloak and shoot him with your rifle.
After doing a certain amount of damage a weak point on his back is exposed, shoot that and then go back to sliding. After hitting the back a few times walls will come down and trap you with him. Don’t worry though as he will then ram through the walls freeing you.
At this stage you’ll have to hit the three circles on his chest to take down his shield, when it’s down do as much damage as you can before the shield goes back up, Salvo can be a lot of help here.
This isn’t hugely hard and if you need to you can go back to the level select and do some grinding to get some upgrades to make it easier.