Dark Soul Crystal is one of the game's resources, and the most limiting one, bar diamonds. Most players (all but the 300 highest scoring on the battle group of the Fighting Pits) will only receive one every three days.
See Fighting Pits sections "Rewards" and "Strategy" for tips on getting as many as possible.
- Tower types and skills at Engineering Quarter
- Magic research at Magic Ward
- Heroes talents and skills at Hall of Heroes
- Improved training at Training Grounds, improved and more supply at Supply Station (1st upgrade).
- Rural Camp Research (Level 30, 60 and 90)
- Researching Elite to Wicked (1) and Wicked to Legendary Equipments (3)
You should probably save DSC to upgrade towers at the start. New tower types are the key to progress in the World. Ditto for the Magic skills. See the comments of Engineering Quarter for some tips on the order of upgrades.
Improved training should probably be next. Once you get to Heroes, concentrate in Hero skills and ignore Hero talents until later in the game, see Honor Star for reasons and related strategy.