From Halopedia, the Halo wiki
Battle Hardened is an achievement in Halo: Spartan Strike. It is unlocked by earning all Gold stars in all missions of Operations A through E. The achievement is worth 20 Gamerscore. It is represented by a circle with a star ringed by five dots.
Unlike its predecessor, Halo: Spartan Assault, campaign scoring does not feature a time multiplier; it is therefore not advisable to rush through a level. Players can maximize their score by focussing on kills and medals.