If you're very new to tactical strategy games as a whole, then keep in mind that by choosing Lone Wolf for both characters, you've made the game harder. With that in mind, I'll give you some build tips, as well as some tricks that you can use which *will* make the game almost too easy sometimes - so feel free to ignore the ones that I say are for difficult situations only.
First, you do not need a tank. The tanking mechanism isn't robust in this game; there's no clear way, for example, to "take aggro" from creatures. So, no, you do not need a tank.
An Archer/rogue build will be one of the less powerful builds of the game, if I understand the meaning correctly. A pure archer can be very powerful, but not from a raw damage perspective; rather, archers have the potential (depending on what arrows you find, buy, and create) to make excellent use of environmental effects. But adding in typical rogue traits will not synergize well with archer. Indeed, I would personally recommend that for your first playthrough you don't worry about the Nasty Deeds tree (which is the typical "rogue" set) at all. The game has so much money floating around that there's no sense in Pickpocket, which is a weak skill anyway. You can break almost every chest that you find in the game; those you cannot, you can find keys for; thus, Lockpicking is out. Sneak is not a bad skill, but Invisibility is significantly better in almost every situation. If your cousin wants to deal real damage, a 2-handed warrior would be the better choice.
Your summoned creatures will be excellent damage sponges; handled well, you should find that there are at most three or four fights in the game that summoned creatures plus crowd control cannot make very easy (if, sometimes, rather long). The spider will do you fine until you get better summons. No matter how pure your elemental spec, spend one point in Witchcraft. Maybe even two; but one, at a minimum. With Lone Wolf you'll have a point to spare.
Following are some tricks that might not be obvious at first, which can make the game much easier for you. I choose not to use several of them because I feel they are too powerful; if you find fights hard, consider using them as needed to get you past sticky points.
The witchcraft spell Blind is extremely powerful; Blind enemies won't do *anything* most of the time, so it's effectively a Paralyze. For four AP, and lasting 3 rounds if not partially resisted, it's hard to beat.
Buffing spells are, generally, more powerful than direct damage. Early on direct damage will do you just fine, but if you will experiment with spells like the Fire spell Wildfire, you will likely find the benefit of casting that spell greatly exceeds the benefit of using the same AP to do direct damage.
Immunity to elemental damage is very useful in some situations. Say you've gotten the Fire spell Summon Fire Elemental. If you cast a fireball (I know, direct damage, but this isn't hypocrisy I promise) onto an oil slick on the battlefield, where enemies are slowed and trying to get to you, you will burn the enemy with damage, set them on fire and damage them from the oil, and heal your fire elemental, all in one spell. But keep in mind that immunity to *damage* does not give immunity to *elemental effects*. 100% Ice immunity will not keep you from getting frozen to a icicle when an enemy archer hits you with a frost arrow.
If you are really having a hard time, get the talents Leech and Comeback Kid on a melee character. When you get hit, you'll bleed; when you bleed, you'll heal; when you heal, you have more than 1HP and if you get hit for a "killing blow" you will be set to 1HP, bleed, and heal up again. Caution: this combo will break the difficulty of combat for you. But so will spamming Drain Willpower + Blind, which can effect bosses in the game and will end up turning combat into "I hit the monster who cannot do anything. I hit it again."
And two general tips to wrap up: position on the battlefield is very important. Do not underestimate the value of the spells Teleport and Featherfall, from the Air school. Finally, take advantage of terrain; use chokepoints to make your enemies funnel in together, blocking line of sight for archers and casters in the back; make them run around a fence to get to you, wasting their turn on movement instead of attack; use oil and ice and ooze (poison) to handicap and damage your enemy.
This game gives you a lot of freedom. Make the best of it, and have fun ^_^
