Sneak, shoot with daedric bow and arrow, or dragon bone bow and arrow.
Usually a one hit deal if you've got high archery skills. Though there's one of them that's a level higher than the usual ash spawn - I forgot the name - but they're two, to three hits. Criticals dependant.
When you're at a low level ranged weapons (i.e. bows) are preferred, especially when combined with sneak attacks & poison, if necessary. Aura Whisper is essential to knowing where the bad guys are before they even know you're there.
Other players might test this out, but I noticed that every time I shot one with my basic Elven Bow ( Superior, but no enchantment ) with Glass arrows, I got one shot kills. Maybe they are particularly vulnerable to glass weaponry for some reason ? Also, Slow Time shout helps for dodging their fireballs and shooting them down, especially when alone and there are three of them... Otherwise, I RUN !
They don't really have any particular weakness but they do have a perk that boosts their melee damage and high fire resist so don't use fire against them and keep in mind they can do a fair amount of melee damage. Dragonborn is made for a character at least about level twenty so if you go over to the island very early (and depending on difficulty) they may be a bit tough, and two years ago Demonprophecy was only just that level.
Don't get to close to them I'm level 35 and I have the glass bow and arrows and try to do head shots from far away or I say for up close you should get a ebeny warax it works like a charm and i say get a bow for it or posin the crawsbow bolts and get hedshots
Ya gotta go in, with two axes, scream, KAMIKAZE!!!! and power attack like crazy. Also, don't forget your potions. Save them for the General Falx Carius. use healing and fast healing if you are on low health. Also, run.
Don't use fire, they resist it, don't bother with frost as they are not weak to it and can easily pelt you with fire at a distance. Best magic is shock, as that will drain their magicka. Wards are also helpful for blocking their magic, bring fire resistance stuff.
Even though Ash Spawn are said to be undead in the lore, they are actually accidentally coded into the game as Dwarven automatons and not undead, meaning they are not weak to what undead are weak to and cannot be affected by Turn Undead spells or other undead targeting effects.
I am a warewolf and whenever I see them I run the other way. lol, but venturing in the nearby place on the way to the temple of miraak you can find a pack of warewolf people, who sells a ring for not less than 8000-9000, which gives you the power to regen health and other rings meant for warewolf perks.
It was easier for me to kill those ash spawns from sneaking and using bow, cause all the other melee weapons felt like waste on them, fire and frost did no good either. so your best bet against them out in the open is hide and stand on an elavated area, where they can't reach you easily and you can dodge their fireballs. and for the close combats, may the blessings be upon you. HAHA