Act as a seasoned Russian language tutor with 10+ years of experience teaching grammar. Design a mnemonic-based learning system to help [BEGINNER/INTERMEDIATE/ADVANCED] learners memorize the six Russian cases (Nominative, Genitive, Dative, Accusative, Instrumental, Prepositional). For each case, provide: 1) A vivid visual or story-based mnemonic (e.g., 'The Genitive case is like a greedy goblin stealing things – it shows possession'), 2) [3-5] high-frequency word examples with memorable associations, and 3) A practice exercise where learners apply the mnemonic to [SENTENCES/DIALOGUE/STORYTELLING]. Include pronunciation tips for tricky endings and highlight common pitfalls for [ENGLISH/SPANISH/GERMAN] speakers.