The Proving Grounds of Cruelty is probably the most annoying dungeon in Fairy Realm. PGoC drops assassin weapon which is good to get for Lynn. Problem with this dungeon is that the mobs have very high Dodge rate, starting with 60% for stage 1 and 100% for stage 4.
What you need to do to beat these stages is to stack up on Accuracy. The easiest way to do this is by: 1. Use Sparrow for his passive (35% Accuracy), 2. Use the Accuracy/Def pen rune set (35% Accuracy), 3. For last step you could either use a Red Rune with Accuracy or use 3 accessories with accuracy. This way you will have high enough accuracy to hit the enemy targets. Depending on which stage you are aiming for you will need different amount of accuracy.