I'm not new to the game (several playthroughs + SoD, BG2, IWD playthroughs on Core Rules) but it's the first time that I try SCS (I'm not putting at the highest difficulty, it's set at Tactical which is sort of the middle difficulty for the mod even though it's a lot harder than Core Rules without the mod).
My characters are now lvl 6, I've managed to kill Drizzt at lvl 5, done the TSC mage island, just killed the Ogre Mage in Firewind Bridge, etc. So I've managed to pull it off through many of the major fights already. Still, I thought that once my Sorcerer would get Spell Thrust, I could take out casters a lot more easily but I was wrong. All it seems to dispell is the Minor Globe of Invulnerability.
Enemy casters seem to generally cast Minor Globe of Invulnerability + Protection from missile weapons + Protection from magic weapons + Mirror Images (and maybe some other buffs I'm not sure).
So for the last few fights (mages on mage island and the Ogre Mage in Firewind), I've noticed that I can break the Minor Globe of Invulnerability but most of my party are physical damage dealers and they can't do anything (maybe I should carry extra non-magical melee weapons?). Ranged doesn't work and magical weapons don't work. I can then land a few Melf's Acid Arrows through their defenses to buy some time by doing a bit of damage with it and interrupting some spells. I mostly then need to wait the enemy caster's defenses out which is not a tactic I enjoy. I've had to do this 2-3 times in my last unmodded BG2/ToB run in very rare cases as I only had an Inquisitor and no arcane casters to break defenses (like vs Kangaxx Lich/Demi-Lich) but it's already common in early/mid game BG1 with SCS.
My current party is the following:
1- Stalker lvl 6
2- Barbarian lvl 6
3- Dorn (Black Guard) lvl 6
4- Blade lvl 7
5- Cleric/Thief lvl 5/6 (getting close to lvl 6 Cleric)
I think I have most of the spells so far that I could find for the lvl 7 Blade (almost done with all the extra zones/subquests, I've got all the best gear I could get/buy for my party until I can access Baldur's Gate city including Drizzt's gear except for Durlag's Tower and the Werewolf Island that I plan on doing later on around lvl 8+).
So for fighting mages, my Sorcerer has Spell Thrust, MAA (no save so it's good for interrupting), Glitterdust (enemy casters also often go invis), Spook (lucky if it can land), Magic Missiles (good vs mirror images but the Protection from Missiles spell blocks it).
My Blade has access to Deafness, Detect Invisibility, Detect Illusion, Dispell Magic, Remove Magic.
Cleric/Thief has access to lvl 3 Cleric spells (so Dispell Magic and Invisibility Purge).
So I think that I need to open with Spell Thrust so that the other dispells can go through Minor Globe of Invulnerability or be able to land some spells if lucky (managed to kill the strong Ogre Mage by landing a stun with a Wand of Petrification). I'm not sure afterwards mostly for the protection from magic weapons and missiles.
I did not activate the component changes flame arrows, acid arrows, frost, biting, etc. so maybe those would work through both defenses?
I can counter invisibility. I would think Remove Magic to be the other best thing to cast to get rid of the other defenses but last time I used it, it never worked despite my Blade being higher level. Either I was fighting enemies that were too strong or SCS raises the level of enemy casters making those spells unreliable? I think I can only access Breach in SoD if I'm not mistaken.
I just want to try to start getting the good habit for stripping enemy defenses right as I plan on continuing this playthrough in SoD and BG2/ToB where there will be even more defenses (but more tools to dispell as well).
EDIT: I just read some in-depth text on how to counter spells with SCS (although it was more focused on BG2). It clarifies a lot (Remove Magic will mostly be bad unless maybe since I tend to clear most of the zones in BG1 and BG2 before I persue the main story, maybe my Blade could be high level enough to sometimes use it). It still isn't totally clear for BG1 though since I can only access Breach in SoD, this means that unless I can land that Remove or Dispell Magic, I think I have no way of removing enemy caster defenses against physical damage (only an Inquisitor maybe could and I don't have one with my party setup).