Army of Ruin
221 Achievements
GOG
Viking Helm
Graveyard I - Reach Experience Level 20 with Ariana
40.8%
How to unlock the Viking Helm achievement in Army of Ruin - Definitive Guide
This will likely be tight on your first run with no upgrades, and you'll almost assuredly need to leave the boss alive while you grind enemies to level up more. It's definitely possible, but you could just as easily come back with some upgrades to make it easier to level up quicker so you can reach Level 20 by the time you reach the boss.