Power StruggleG 15

If you're in this game for the long haul, just wait until you hit level 60 to do this achievement.

When you hit level 60, wait until the first board gets invaded. Once you have that, make a deck with 3 level 1 cards and 1 5 star card. If you want to use any runes, use runes that lower the opponents atk only. Not trying to kill the opponents cards off but slow down the damage they do to you.

Once you have all that set in place, go fight the invasion. Play your 3 level 1 cards to take some damage instead of your health getting knocked down. No worries if they die, they are just to help slow down the damage done to you. Save your 5 star card and don't play it. With the amount of health you have at level 60, just let the rounds go by until you pass round 50. Once you pass round 50, play your 5 star card to kill the opponent off. Achievement will pop right up once you win.