Your Character
If you complete all of the available side quests, then you should reach level 13 by the end of the campaign.
Be sure to use "expert mode" when you create your character. Reduce the prowess of any weapon talent you won't use, and consider picking a disadvantage for a group you won't use. For instance, if you will never use the character for crafting, instead using followers for that, consider picking the -2 on all Artisan talents disadvantage. Advantages are also worthwhile, particularly Regenerate AE, VI and ED are must-have for mage and warrior classes, respectively.
The game's companions have been designed to (initially) form a well-balanced group regardless of the PC's talents. Use and expand their specific skillsets, for instance specializing Cano in socializing and thief skills, Fayris in healing and Forgrimm in tanking in combat.
Rogues
There isn't any downside to robbing people, so try to pick pocket everybody.
If you don't have any lock picking tools, then you'll take a 10-point penalty every time you attempt to pick a lock. So make sure you keep your rogue supplied with
Hair Pins or other tools. You should get a locksmith's knife early in the Complete Your Training quest, keep it on you at all times.
For deonts of Phex, do not worry overly much about spending your miracles. Use Starscatter frequently in combat as the first level generates very fast. Divine Intervention's duration is simply until the number of tests it is cast for run out, so there is no reason not to always have it active.
Spellcasters
You won't be able to cast spells if you're wearing metal armor. So keep your spellcasters in leather or cloth (or wood, in the case of shields).
The Clarum Purum spell will cure all poisons, even if you never put any points into it. Plus, once you've learned it (probably in Chapter 3), you won't need to worry about the Treat Poisons skill for any of your characters, and you'll no longer need to hang onto
Golmoon Tea.
The Balm of Healing spell can remove wounds, but only if the number of wounds is less than or equal to the modifier on the spell. That means you'll need to keep and use
Simple Bandages (or other Treat Wounds items) for most of the campaign.
Miscellaneous
The ship Thalaria serves as your base. Each companion has his own traveling crate, it is advisable to use these to store crafting items, and I personally preferred to split up each talent's crafting items over the chests. Once Ardo is with the group you can install crafting tables on the ship itself though I did not consider this particularly wortwhile.
All characters can use shields.
During battles, you should always order your entire party to attack the same enemy. Enemies slowly regain health, so it's good to kill them as quickly as possible, and they can only parry at most two attacks per round of combat, and so the more attacks you send their way, the better off you are.
If you're short on cash, craft. A simple item like a stag beetle dagger, which takes only Mandibles and Leather Strips to makes, can be sold for more than 1D.
The game is laden with in-dialog talent checks. Even though they're not always necessary to complete a quest, they always net you AP for succeeding at them. The amount of AP you get is proportional to the difficulty of the check.
If you activate
Simple Bandages (or other Treat Wounds / Treat Poisons items) from your inventory, then you'll only be able to use them on yourself. But if you activate them from your quick slot bar, then you'll be able to use them on anybody in your party.
To clear a hotkey, just click the middle mouse button on it.