Difficulty is a synonym of Obstacle. Hazard is a type of Obstacle. Hazard is related to Smoke. Smoke is related to Dust. 
Pool can be used for Drown. Drown is a type of Eliminate. Screen entails/necessarily involves Eliminate. Blind is a type of Screen. 
Difficulty <--Synonym--> Obstacle <--IsA-- Hazard <--RelatedTo--> Smoke <--RelatedTo--> Dust 
Pool --UsedFor--> Drown --IsA--> Eliminate <--Entails-- Screen <--IsA-- Blind 