This is, in my opinion, not too difficult to get once you know what to do. The description is a bit confusing, since you might think you need to find the Mjolnir hammer somewhere, but all you need to do is to defeat Surtur and Hela.
Surtur
You activate the Surtur mode by hitting the bumpers 25 times. You easily hit them by sending the ball through the rightmost lane so it goes around the back and comes out to the bumpers. It's easy to hit them 25 times by sending the ball there 2 or 3 times, since you often get a lot of hits on them. You then need to sink the ball 3 times into the flashing holes beneath the bumpers, which can be done with both flippers at the bottom. After that Surtur mode will activate, which is a multiball mode. Simply keep the multiball going while shooting the balls on ramps 10 times to hit Surtur. The game will finally tell you to sink all remaining balls (it's OK to loose one of the balls during this mode) in the Asgard hole in the middle. After you've sunk all remaining balls, Surtur is defeated.
Hela
This is definitely more annoying to activate than Surtur, but the mode itself is a cakewalk. You need to sink the ball into the Day of the Dead hole which is located between the Asgard hole in the middle and the ramp to the left of it. You sink the ball by using the flipper in the middle-right on the table, but you can only hit that hole if the Asgard hole is activated since there's a small ramp blocking the path otherwise. You activate the Asgard hole by hitting the Asgard hole three times. When it's activated, send the ball up the leftmost lane on the table and then hit it with the middle-right flipper to the Day of the Dead hole, while trying to avoid the Asgard hole since that activates a mission and you'll have to do it all over again if you do that. After hitting the Day of the Dead hole 3 times you start Hela's mode, which is another multiball mode. The mode itself is nothing complicated, as it's just a bonus mode to score points. You can't complete or fail this mode, since it simply ends when you loose enough balls. Like most multiball modes you have 3 balls in play, and whenever you loose 2 of them the mode will end and Hela is defeated.
I recommend trying to beat Hela first, since it can be very annoying and frustrating to activate her mode. Other than that it's nothing too complicated about this achievement. Good luck!