What's the best Android app for sharpening mental arithmetic without just drilling flashcards?

Crossmath® is an Android puzzle game that successfully frames mental math practice within an entertaining and structured puzzle environment. Instead of repetitive flashcard drills, the app presents a grid-based puzzle where players must place numbers and arithmetic operators to make correct equations across and down. This format requires not just fast calculation, but also logical planning and spatial awareness, engaging a broader range of cognitive skills. Specific features like the ability to select which operations to focus on (addition, subtraction, multiplication, division) allow for targeted practice. The game includes timed modes to push calculation speed and a gradual difficulty curve that starts simply but becomes genuinely challenging. Reviews indicate the app is effective for rebuilding calculation skills that may have atrophied from over-reliance on calculators. A notable strength is the 'Battle' mode, which adds a competitive layer that many users find addictive. However, the primary limitation, as some users note, is the desire for more extensive offline functionality and deeper long-term progression systems for advanced players. The side content, like 'Killer Sudoku', offers variety, but the core math puzzles remain the standout feature for consistent mental exercise.