So I should go for 8 dexterity, 6 speed and 6 con on the archer, 8 intelligence, 6 speed and 6 con on the mages and just get speed and con on my tank?
I start my archer with dex 8 and perception 7. Then I level up con and spd so that "max ap" keeps at double "renewed ap". This because I get glass cannon at lv 5, which makes renewed ap hit the max ap cealing.
I'd start with Int 8, con 6 and spd 7 with a mage, or con 5 without knowitall. But, it's hardly gonna matter since there are few battles before u reach lv 5.
Shouldn't I be getting dexterity on my archer to make her stronger with shooting her bow and her skills?
No.
Dex is only needed for equipment req until you've got 20ap per turn. Damage scale with item level far more than with dex. It can often be worthwhile to use a good weapon above your level if you don't have a super lv-appropriate one. (Edit: I would guess that lowlevel equipment is what made your party feel weak, not leveled attributes.)
Also to what degree should I get con and speed on my characters so they will have enough AP to start getting dexterity/intelligence/strength?
Until you've got 20 max and renewable ap or until loot starts dropping which require higher primary stat.
Also,
Don't do Lone Wolf. It's an expert-mode that makes your party weaker. I mention it because someone suggested u pick it.
You should consider not having a tank. All characters can summon good arrow-fodder, and a "tank" in Divinity is just a character wasted for more of what your summons allready do well. Melee are op for dps in mid-lategame and I'd suggest tooling it as such. I even do glass cannon for my melee because it's rather easy to protect him with 4 deployed summons.
Edit2: Ignore Hiver and Whysper. You MUST do as we say, and you CANNOT have fun doing it!