Another easy achievement.
I did this after hitting level 10. On the world map, open the menu and go to invade. This will put you in a pvp match against another gamer. It definitely is a little bit luck based. It depends on what kind of enemy you get and what troops set up he has. _Use the best troops you have and, if you can, level your troops before you try this.
Just a side note. I got this achievement after winning the first online match, but I did not get a achievement notification. However, it was unlocked right away.
Here some details on my personal set up with which I unlocked it:
Hero: Lvl 10 - Hunter's spear - attack = 4 - Defense = 6 - health = 10
Night Terror: lvl 4 - defense = 2 - health = 6
Boar Rider: lvl 1 - defense 1 - health = 4
Serpent: lvl. 2 - defense = 0 - health = 4
Hope this helps!
Any improvements? Please let me know in the comments below!