Even though the game is free, it is by no means a waffle. If you are looking for an easy completion, I would not suggest downloading it. I know of one achievement that will correspond to a new mode to be released later, so its likely that the other 2 modes and areas will carry more achievements with them, so even if you take on the challenge of launch content, be aware that more is on the way. I should also mention that paying for souls may help progress through the game in general, but will not get you all the achievements without effort.
Your ultimate goal is to get the Transcend achievement. You should have every other achievement by the time you get it. In order to transcend, you must reach level 60, which requires 8 ascensions at minimum. During my 2 months of beta play, I ascended 4 times twice. This achievement will not be easy, taking between 50-60 hours for that single achievement!
Since you need to ascend so many times, I would suggest doing do every 5 levels and try to switch gods just before gaining a free spell. This will get you the carving a path and champion of the gods achievements easier. In my experience, the void gods first spell seekers, the dark gods second spell impale, and the light gods third spell lightening shield are best. Thus I would suggest aligning to those gods in that order.
Be sure to use any crusade spells before you ascend since you will lose them. You will need to banish 25 enemies, bless 20 players, and curse 20 players for 4 achievements.
Speaking of spells, the power overwhelming achievement requires fully upgrading a spell. The cheapest to upgrade is a level 1 spell, with the last upgrade costing 15k. I would suggest putting your EXTRA souls toward this, but be sure to keep some for maintaining equipment. You will find so much new equipment, its not worth buying any from the store. Once a spell is maxed, the nuker achievement for killing 25 enemies with magic will come much easier.
Going beyond the first 3 ascensions, I would stick to the dark god for the damage and armor boosts to make getting the outfitted for war, equipping a 200 damage weapon, and caos supremacy, equipping a total of 2000 armor, achievements easier.
The last bit of info I can give is the requirements for the secret achievements:
completing the highlands story by defeating the titan and
by finding all the lorestones in the highlands
and the achievement to be added later:
Dungeon Master- complete 50 floors of the endless dungeon
I hope this will help someone decide to attempt to complete it or point them in the right direction to do so. I don't get on XBA often, so if you have a question, you should message me on Xbox Live to ensure reply.