Welcome to the flock.
I can't / won't give you an explicit building guide, but some general tips that may be more or less obvious.
First and foremost, D:OS let's you build freely without classes or anything like them. That said, some decisions may limit others, the most prominent being your attributes that limit the skills you can have in a particular skill tree. E.g. for higher Aeromancer skills (mage) you need high intelligence, for higher Marksman skills (archer) you need high dexterity. Many talents also have necessary attribute or ability levels.
This freedom also means that you don't have to make the "perfect" characters right from the start. You can let them grow into other directions later and even dabble a bit in other professions' skills, as many Novice skills don't have very high attribute prerequisites. (Disclaimer: This applies to Normal difficulty, higher difficulties may require a more strict character build.)
Aero and Hydro work well together, as water can be electrified.
You'd also get a healer which is always a good asset and can be game changing in every stage of the game. That said, most elements interact with one another. Fire can evaporate water or ignite poison, for example. A little warning about your plans for a Aero/Hydro mage, though (black on black text because of a mild spoiler, mark to reveal):
In the first city of the game, you can find a companion NPC who is Aero/Hydro, so you might cover other elements with your main PC mage. But since you can only take two companions with you at one time, you might as well keep your own Aero/Hydro mage. Just so you know ahead about this.For the archer, the Ranger "class" is a good start. Since archers mainly work with elemental and other special arrows, you don't have to pay that much attention to their skills for that like you do have for your mage(s).
I'm closing with a little excursion into crafting: Some people externalize crafting to NPC companions to safe the relevant ability points of their main characters' builds. My co-op partner and I have divide the two crafting abilities "Crafting" and "Blacksmithing" between our main characters, mostly out of roleplaying reasons. Another reason to do that is that you don't lose them (temporarily) if you swap out your companions. However you want to do it, think about it at the start of the game and modify your characters' abilities accordingly.
Have fun with this great game.
Raban
