...seems like they only will stay down when you cut their heads off, but is there a way to do so? I tried to use varied angles, I tried to use different weapons but it always seems random when and if at all lose their heads.

It takes a bit out the fun and is really tiring when you hit one of those 2 times and he's dead and for another you have to hit him 20, 30, 40 or even more often times -____- Not to mention the time you have to wait till they finally get up again as long as they're not killed.....

The game really was fun until this stupid gameplay decision point now.....

I agree, the skeleton guys are annoying to fight. I bought this game a few days ago and I too am in the hell levels. Unfortunately, there is no real strategy I know of to defeat them easily, other than "jump attack with a sword repeatedly until you get lucky". The GameFAQs walkthrough doesn't give any helpful advice other than to use the sword. You have to hit their head to kill them, so it doesn't matter how many times you knock them down, you have to keep fighting them until you knock off their heads. Just try sword+jump attack. It might be clunky but its all I can think of.

If I remember correctly, the Roman Sword has a sideward swing that's easy to perform and hits at just the right height.

Guess we better just persevere until we find a Roman Sword then. Thanks!

Besides the Roman Sword and the sword jump attack, I seem to remember two other ways to kill them. Fire is one - try to keep a torch with you until you get the Roman Sword.
The other is to hit them with the second swing of the Viking Short Sword, which hits at head height. This can be tricky because the first swing will knock them down - you need to swing while they're standing up so the first swing doesn't register, but the second does.

Prior to the Roman sword, the hand axe works wonders on the undead skeleton warriors; just get up in their face and swing!.

Bit late, but the Roman Sword makes taking out the resurrecting enemies much easier. You can reliably kill them by standing with them on your right side and slightly in front of you, then just swinging once. More often than not, it takes their heads off straight away.

You can also do a downward attack at heir heads/necks with most weapons when they're knocked down. It's harder to pull off, but it does work.

The zombies which keep getting back up will stay down if you hit them with fire, a single blow from either a rune empowered handaxe, or a flaming torch will put them down for good.