This achievement is unlocked by purchasing all available upgrades on all 4 weapons, all spells, and health and magic bar upgrades. The easiest way to get this with the least grinding, and get most of the other achievements in the game along the way, is to do the following:
1) Play through the game on Easy difficulty
2) Play through the game on Normal difficulty
3) Play through the game on Hard difficulty
(There is no benefit to playing first at higher difficulties because the difficulty achievements are NOT stackable)
4) Start a new game on hard difficulty and play through Level 1, then Retire from the game. Start another new game on Hard difficulty and grind Level 1 again. You should get 350 upgrade points every time for an S ranking. Repeat until you have enough points to complete all upgrades.
By the time you finish with this you should also unlock 6 achievements for Stage Completion, 3 for Difficulty Completions, 1 for All Stages Cleared and probably 1 for 15 Hours Played. Now you are ready to take on the last two achievements for Chaos and Hell difficulties.
The first 3 difficulties are easily beatable without full upgrades. You will definitely want to have full upgrades for the last two difficulties though, so good luck!