To get the achievement, you need to be carrying 30K techscrap and beat the boss.
It seems like you need to have the scrap on you from the beginning. I tried to get round it and dismantle scrap piles and caches in my inventory just before delivering the killing blow to the boss. Doesn't work. 🙁 I tried with Delver Echo Alpha and with Goddess Helena. I had 70K of scrap on me when I finally killed Helena and no cheevo. 😥
Don't sweat it too much. I got it for Archangel Eli, the final boss of the game. The good thing is that when you die, your scrap stays not in the boss arena, but on the platform BEFORE the arena, so there's almost no risk of losing it... if you don't die on your way from the med bay to the platform, that is. But even if you do, getting 30K scrap on the great wall is not too much work.