Conan

Very easy to do. Fight the squid normally. It will start to change your crew into oilmen. Just kill them... simple as that they are VERY weak. Might think about doing death rain (throw 100 enemies to their death) and the 50, 250 and 500 grapple kills here as the enemies will never end.

For this achievement, it asks you to kill at least twenty-five enemies during the Giant Squid boss battle.

Take on the Giant Squid and fight him as you normally would. He'll then switch tactics and start changing the crewmen into oilmen. When that happens, ignore the tentacle assault and work on killing any and all oilmen that you see. Just attack them as you would normally would on any enemy until you performed your twenty-fifth kill on that twenty-fifth oilman. Once you destroyed that one, the achievement should unlock right then and there. Be careful not to hit the tentacles too much or else you will miss your chance. If that happens, just reload your game and try again.

Have fun and good luck!