-Estimated achievement difficulty: 2/10
-Offline: 8 
-Online: 5 (250)
-Approximate amount of time to 1000: 20-25 hours
-Minimum number of playthroughs needed: 1
-Number of missable achievements: None
-Glitched achievements: None
-Do cheat codes disable achievements?: No Cheats
Phantasy Star Universe is an RPG with a free-action battle system. It is one of the shortest and least involved RPGs available on the system. The original 1000 points can all be obtained through the single player storyline, while the additional 250 require the monthly paid service that acts as an MMORPG.
Step 1: Original 1000
To obtain the full 1000 in this game, you need only play the main storyline. No extra exploring, leveling, questing or item acquiring necessary like more recent RPGs. Once you finish the game, you will have your 1000 gamerscore.
Ambition of the Illuminus DLC
If you aim for 100% completion of this game, you will need to subscribe to the online service (monthly fees apply) and go on various quests with multiple allies to defeat difficult bosses. Since very few people in our community have done this, there is little help to be found, but check the forums or search on places like Gamespot for help if you really want to do this. I do not know how long it would take to get these achievements, but I know MMORPG games can be quite tedious and difficult if you can't find good players to help you.
[XBA would like to thank The Pants Party for this Roadmap]