Here is the full list of all 45 Final Fantasy VIII achievements. It takes around 100-120 hours to unlock all of the achievements on Windows.
Reach level 100
Unlock Guardian Force Doomtrain
Reach maximum HP
Unlock Guardian Force Bahamut
Draw magic 100 times from draw points
Complete the PuPu side quest
Complete the Obel Lake side quest
Unlock Guardian Force Cactuar
Find Ragnarok
Reach SeeD rank A
Defeat every member of the CC Group
Unlock Guardian Force Alexander
Unlock Guardian Force Cerberus
Unlock Guardian Force Tonberry
Acquired an item from Chocobo World
Capture a Chocobo
Unlock Guardian Force Pandemona
Unlock Guardian Force Leviathan
Unlock Guardian Force Carbuncle
Upgrade your weapon
Unlock Guardian Force Brothers
Win 100 games of Triple Triad
Draw (Stock) magic from enemies 100 times
Unlock Guardian Force Diablos
Get your first salary from SeeD
Unlock Guardian Force Siren
Kill 100 enemies
Lose a rare card in a Triple Triad game
Unlock Guardian Force Ifrit
Unlock Guardian Force Shiva
Unlock Guardian Force Quezacotl
Play Triple Triad
Unlock Guardian Force Eden
Collect all cards
Reach maximum Gil
Finish the game
Finish the game with Squall's initial level
Beat the Omega weapon
Collect all Timber Maniacs magazines
Collect all Weapons Monthly/Combat King/Pet Pals/Occult Fan magazines
Reach level 100 with Boko
Learn all of Quistis' limit breaks
Learn all of Rinoa's limit breaks
Kill 1000 enemies
Kill 10000 enemies