So you first need to get to the clouds level or any level with higher ground. Then you stand on the higher ground and your sword on the lowest position so it looks like this:



And then you can go and do something for an hour (Nidhogg lied, so either get your pitchforks and demand a change in the description or wait the hour) or more while the CPU mindlessly runs into your sword of doom and dies a lot, working towards the "Bloodlust" achievement. And since he can't kill you, you'll also get "Trapped in Donkeyspace".

Do a local multiplayer and choose a level, any level. Make one of the players constantly run into another's sword by putting something on the key to make it stay in. The running character will run rapidly into the other person's sword, rapidly killing them. You can leave now while this happens.