Core Scramble (2010)

Blurb:
In a world plagued by monsters swarming through portals from other dimensions, a brave group of soldiers called Clarus Orbis battles the otherworldly threat. Unfortunately, when you’re an average grunt like Chaeun, survival becomes a daily ordeal, especially under the reckless leadership of his sexually arrogant commander Gayoon. Abandoned on the battlefield by the handsome Gayoon, Chaeun desperately fights off an endless bug onslaught only to be rescued by the mysterious renegade leader Moonhoo. At first, the man playfully flirts with Chaeun, but slowly the realization dawns that Moonhoo and his Core Hunter group have insidious ambitions on the dimensional portals. And only Chaeun and his hot-tempered leader Gayoon stand in their way.

Source: www.anisearch.com/manga/14436