Dude, I finished the game every time after the first try on the most difficult difficulty. With zero NPC deaths on one quest (the defense of Redcliffe) every time, and defeating a certain battle in a certain part of the story you're supposed to lose, every time.
I'm thinking you're not doing "tactical" right. Rogues are great for tactical. General principles for combat are:
1. Have your tank character engage (at the start could be a sword-and-shield warrior, or a certain pet, and depending on your playstyle, an arcane warrior) and get aggro. Make sure you always do this first. At the start you'll have to make sure everyone else stays behind until all the enemies' attention is on your tank, until your tank gets skills that draw aggro to him. This is the most important part.
2. Second, if you have anyone with any crowd control skills (stunning skills, knockdown skills, paralysis skills, sleep skills, etc.) use them on whomever isn't wailing on your tank. This the second most important part.
3. Put your other characters in a position where they're most effective. Secondary tanks like two-handers can engage stragglers or rush backrow attackers like mages and rangers.. Bow rogues/warriors stay behind and do their stuff. Damage-focused mages and healers stay behind, too. Some of them may have crowd-control skills, remember to have them use those BEFORE trying to deal any damage.
4. This was supposed to be in 3, but since you like rogues, I'll put it here separately. Rogues backstab. Literally backstab. As in you go towards the enemy's back then stab them. This is where they're most effective and most fun. Make sure you kill the enemy before they notice you, because rogues are squishy as hell until the late game (and when they notice you, you'll never be at their back for a backstab, severely stifling your rogue). Later on you'll get skills to disappear, but aggro partly depends on damage dealt, and rogues deal A LOT of damage. You may want to run back and hide behind your tank friends if your rogues gets more attention that he can handle. In the early stages, I tend to have rogues backstab once or twice then run around while the enemy chases him to get some heat off the tank.
5. Repeat all over again. I strongly suggest befriending the pause button. I usually have AI for all my characters off, because I like micromanaging them.
As you noticed, the key is crowd control. You want as much of the enemy unable to do what they're supposed to do for as much time as possible before trying to kill them. That's why you need to make sure your enemies are wailing on your damage sponge tank instead of everything else.
So how do you build your characters:
1. Tanks should focus on ways to attract the enemy to them, then ways to reduce the damage they take, then ways to crowd control (like knocking them down or stunning them).
2. Rogues are meant for one thing: To kill quickly. Make sure they can do that in one to two hits. Killing something in one hit is also crowd control. Bow rogues/warriors can also do the same thing, but a bit less efficiently, from afar. Everyone else in this list does crowd control, so the rogues generally get the share of the killing.
3. Mages generally should go for crowd control above everything else. There are good skills that do crowd control and do great damage. I'd put healing at secondary. Then go for certain spells that inconvenience certain types of enemies pretty well (Particularly noteworthy is that mana burn spell that one hit kills most mages, including bosses). Mages are the reason why I turn AI off. A lot of excellent AoE spells (cone of cold, for example) are much better with you casting them yourself.
4. Secondary tanks like two-handers should go for crowd control abilities first, then damage second.
Finally, individual battle strategies are up to you to figure out. If the enemies frontline is weak, you may find it best to smash their frontline first then going for the back. If they have glass cannons in the back that are problematic then you may want to rush them first while having your tank take punishment for a little while. This is the fun part, so I'll let you handle that. DA:O is a very tactical game that for most battles figuring out the strategy is the key to winning.
I hope this helps. DA:O is a great game, and it deserves a good look into it.
EDIT: Last note, if you can get combos for instant kills or great damage, go for it. Note that rogues auto-backstab when the enemy is paralyzed or stunned, so getting an enemy stunned then stabbing them to death with your rogue is a VERY good idea. You can also do shattering combos. Frozen or petrified enemies will shatter when hit with a critical hit. Have a mage freeze or petrify someone then either have a two-hander or bow-user do his critical strike or have the mage use the stone punch thing and the enemy will be insta-killed.
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