Balancing difficulty levels

1. Skill-Based Progression vs Level-Based Progression (Gamification Player Progression Tips)
2. Player Competence vs Player Performance (Gamification Player Progression Tips)
3. Player Experience vs Player Engagement (Gamification Achievement Systems Tips)
4. Player Improvement vs Player Progress (Gamification Player Progression Tips)
5. Player Skills vs Player Abilities (Gamification Player Progression Tips)