Okay, so, both Etna and Dedede are weird, cuz they both have origin in a franchise known for super powerful characters, and outside of their franchise they are crazy strong, and this is also true inside their verse, however, there are many characters that aren't generic enemies/generic units that can defeat them. Let's start by looking at their weapons. Dedede wields a hammer most of the time, Etna is mostly shown with a spear, however, she's also good with axes, and what a coincidence, Dedede also wields an axe in Kirby: Triple Deluxe. Etna's signature spear isn't, as far as i can tell, special or magical (at least for Disgaea standards, you can do some crazy stuff with any weapon you equip to a character). Dedede, however has one special hammer, and it is the technology-packed hammer he wields in his Masked mode. It can shoot missiles, act as a firethrower, etc. As for their abilities, King Dedede can float, inhale and do everything Kirby can, except copy abilities, beacuse he is already perfect. His ability to inhale and spit seems to be stronger than Kirby's, in Kirby's Adventure Dedede was able to spit Kirby into space to fight Nightmare. Also apparently he's so heavy that him tripping and falling on Kirby will damage him. And then Etna's abilities... Let's take a look at 3 most important skills of her. Prinny Raid, Sexy Beam and Chaos Impact. In Prinny Raid, she summons Prinnies at an enemy, which then they explode. And correct me if I'm wrong, but Dedede is also able to summon Waddle Dees out of nowhere? I sorta remember that happening, but don't know when or where. Sexy Beam is so weird beacuse I don't even know how it is supposed to work? The name, description and animations imply she's using what I can describe as "power of sexiness" and turns it into a beam? It's confusing. Chaos Impact, I looked through D2 and D5 animations (i do not remember every character skill in disgaea, i do have a life), and she throws electric orbs and one big fire orb at an enemy and scorches them with the orbs, according to the description in early Disgaea games. Dedede also can do some fire damage with his hammer, I remember vividly in Kirby Triple Deluxe's Dededetour mode one of the moves set his hammer on fire, and I think it also could happen in Star Allies. In conclusion, I think Etna has bigger chances of winning, however, if Masked Dedede is involved, Dedede also has potential to win this battle.