+ Estimated achievement difficulty: 8/10
+ Offline: 575
+ Online: 215
+ Either online or offline: 210
+ Approximate time: 150 Hours
+ Minimum number of playthroughs needed: 1 (but parts of the story have to be replayed multiple times)
+ Glitched achievements: None
+ Cheats: None
This is a long game with some frustrating animation sequences that cannot be skipped, but if you enjoy Magic the Gathering or Monopoly, you may find this to be quite the entertaining game. The multiplayer aspect is quite fun if you can find an opponent, and with the wide variety of maps there is enough to keep you going for quite some time.
The single player achievements are largely for beating the story. Note that you cannot generally save the game in the middle of a battle and still earn achievements when resuming later. This is important in the later part of the game where battles can take 4-6 hours, so plan accordingly.
The starting deck is utter garbage, so your best bet is to play (and probably lose) the first battle so that you get the starting deck, and then move on to multiplayer so that you can earn cards and build a better deck.
Once you have a deck worthy of anything, progress through the story mode. After the final battle, you need to use the "Load" option on the main menu to go back into the story mode and do the rest of the single player achievements. Note that a strong neutral deck can generally win any of the story modes if you don't want to shape your deck each round.
Under no circumstances should you ever select "New Game" once you have started the story mode! This deletes your progress toward multiple achievements!
Specifically, replay Santana Village vs Goligan, Dinar Mountains vs Zajaran, and Grave Canyon vs Zenith. You also need to accumulate 100 victories against AI opponents in story mode, so keep replaying missions (Santana Village is usually the fastest, although it can get boring doing such a simple map over and over.) You also need to collect all the Avatar parts, many of which can be done at the same time as the 100 victories. Many of the avatar parts are easier to do on some of the non-story MP maps. Select "Versus" and don't change anything on the first screen, select a map, and then choose the map and AI cepters you need. The forum has a thread to help with specific avatar parts, which can be found here
Most of the multiplayer can be earned with just one partner. Play on Mt Etumer and avoid shrines and temples. Set it to 1 min turns, 10 rounds, and make sure that neither of you score any points (don't capture lands, etc.). This will make it a tie, which counts as a win for both of you. You will both win 12 cards and make progress toward the 200 wins achievement. It takes a while, but this is a great opportunity to work on the card achievements. That should get you almost every card in the game and make the single player experience much easier and more enjoyable. There are 253 creature cards, 78 item cards, and 156 spell cards, for a total of 487.
Along the way (without interrupting the win streaks of course) you will need to play a 3 player match, a 4 player match, and a 2v2 alliance match. Note that you do not need to win, just play them.
The hardest part is definitely the avatar pieces, so worry about that one last. You definitely want to have all the cards for that, and a solid understanding of the finer strategies of the game.