This achievement can be a little challenging. For more lives go to the main menu then help & options, then settings, then change the extend setting to "1st 10,000pts to Every 40,000pts". Try get as many special flags as possible and try to reveal as many sol citadels as possible. You get 2000 per sol citadel revealed and another 2000 for destroying it. That's the best advice i can give you. Just keep practicing and you'll get it eventually.
taken from http://www.xbox360achievements.org/game/xevious/guide/