A few things for you:
1. Always send your tank in first. Like others have stated.
2. Never use the "defend" option for compainion AI targeting tactics. EVER. It's broken and your defending party member won't defend themselves at all. Always use the "Follow" option instead. It is more or less the only remotely fleshed out AI script available and they shouldn't have even let the player choose AI at all. Sales tactic I'm guessing.
3. You DO NOT have to do any gate closing early on which constitutes 90% of the hardest hitting magic using enemies in the game and magic bypasses armor. Gear up with magic resistance upgrades and health if there's no other option. Then go steamroll the gates later on while on any difficulty level. Nearly all gates are optional to the main story.
4. Take the heal over time potion untel you get more familiar with the game mechanics. Don't feel bad because Bioware does garbage job of explaining the mechanics in this game. Use it as much as you want in and OUT of combat untel you get your head wrapped around the odd mechanics. There is more than enough elfroot in the game to use it after ever combat for healing up your party to full. UPgrade this potion ASAP all the WAY. The last upgrade will save tons of elfroot if you get sic of picking it all game. Think of it as Bioware's idea of a regen system while not in combat but now you gotta work for it a tiny bit. Why on earth it doesn't cost you for FULL insta heals is beyond me. Hella wonky system for sure.
5. As far as the armor mechanic goes. It's nearly entirely based on difficulty level as to how much you can rely on it. Just stay away from hard and nightmare untel you get the deaths under conrol. Keeping your armor upgraded isn't nearly as important on normal and lower difficulties.
6. Abuse the gaurd system on your tanks. Gaurd makes use of your tank's armor values and defensive bonus's from upgrades. Gaurd is MUCH less affective on nightmare because of this. ( I won't get into why right here cuzz this post will get too long)
7. Abuse the hell out of the barrier system. Unlike gaurd it doesn't use bonuses from armor BUT you get A HUGE amount of it. Fully upgraded it's somewhere over 3 thousand which makes a nice buffer and works MUCH better at protecting agains MAGIC users as opposed to stacking Gaurd which will do absolutely nothing if you have no magic resistance upgrades on your tank. No I'm not gonna tell you what upgrades you should take specificly but just know that most of them are fairly well ballanced between each other. Stay away from bleed and armor pen upgrades at anything but lower levels. This is the only tip I'll give you in this regard because much of what you take is going to rely on how you like playing. (bleed works like an armor pen which is timed. yea I know right?)
If it helps. I play on nighmare or hard and never use gaurd or barriers anymore. No, this isn't a big fat BRAG. I'm trying to point out that there is HOPE down the road once you understand how everything works. So don't give up 
EDIT: One last thing. You WILL run into areas that your party WILL get WIPED on. Unless you're keen on upgrading defense to suit EVERY situation, It's going to happen. Regardless, your first time through the game when you aren't familiar with different enemies and if you don't check defenses when you run into a new one... you're likely gonna get killed OVER and over. Woulda been nice if they showed some info about attack damages on enemies but honestly the way the mechanics work... I think it woulda caused even more confusion.