Get A Life

A+ on every chapter - the toughest achievement here and likely your last. The grade rewards speed, big combos, varied weapons/kills and boldness, so each chapter wants a fast, aggressive, high-combo, mixed-weapon clear. Some chapters are far harder to A+ than others (the boss and gauntlet levels especially), so look up per-chapter tips for the nasty ones and expect to grind them. Restarts don't count against you, so retry freely until the run clicks.