Strange Brigade

This is story related and cannot be missed.

After reassembling the Shield of Bes and unlocking "Is that the bes you can do?", Seteki appears to exact her revenge.

After battling her and her two clones, she summons the Titan of the Damned, who activates four electrified zappers. To defeat the Titan, you need to destroy each zapper. To do so, there are four red circle floor plates found around the inner chamber guarded by spikes. When you've eliminated enough enemies, the spikes disappear on one of them, exposing a zapper. Stand in the circle and shoot at the zapper that was just exposed. Repeat for all four zappers to defeat the Titan.

With the Titan eliminated, Seteki then reappears with 8 clones rotating around her. Once her clones are defeated you need to defeat her once and for all. Finish her off and the achievement will unlock.