No need to farm whatsoever. Just end one level with 100+ souls. Then as soon as you start a new level just upgrade one of your stats and die. You'll continue with the 100+ souls you started with before you died AND you'll have the upgrade.
Okay, the last epic level is perhaps a bit unbalanced: impossible without stop, and with stop maxed you can beat anything easily. But is that a good enough reason to give an otherwise perfectly nice fun game a 1/5?
Bah, I hoped for perhaps something nice, as I finished the story for the 3rd time. However, the same "you have finished challenge mode, you get 100000 gold" text line came and the cash never came...
 "Epic War 5"){kind=icon}
 "Elephant Quest"){kind=icon}
 "Tyrant"){kind=icon}
 "ClickPLAY 3"){kind=icon}
 "Soul Redeemer"){kind=icon}
 "Shoot Pixels"){kind=icon}
 "Epic War 4"){kind=icon}
 "Fly Or Die Multiplayer"){kind=icon}
 "Enough Plumbers"){kind=icon}
 "Skyfyre"){kind=icon}
 "LRD: Rebirth"){kind=icon}
 "DSDefense"){kind=icon}