This is a story based achievement and
CAN'T BE MISSED.
SPOILERSYou will unlock this achievement during the mid to late stages of Disc 3.
*** Spoiler - click to reveal ***Once you are launched into space (again, story-based), you will arrive on the first ship and have to go through all this dialogue, meet Ellone and eventually Rinoa will become possessed by Ultimecia and subsequently release Sorceress Adel. Rinoa will be left floating in space and Squall and a party member (you have chosen to go into space with) will head off in a pod with Ellone. After some further lengthy dialogue you will control Squall and have to float towards Rinoa in space and 'catch her' (Don't be phased if you fail the first time, you can keep trying........just keep Rinoa in the centre of the screen and you will be fine). Once you catch Rinoa (and after more dialogue) Squall will see another ship floating in space and will start heading towards it. This ship is the Ragnarok. The achievement unlocked for me once Squall had landed on the outside of the ship and was trying to get inside.
Happy achieving!