I have 4 Questions

1. When Aloth joined my party he had a Grimore with set spells. As the game progressed we have come across several mage books with spells he doesn't have and is able to copy them. Now here is my question: How do you remove a spell in his Grimore (without permanently removing it from your book) and replace (unlearn) it with another one that you have successfully copied and utilize them in game?

2. What have you found to be the best armor and weapons for your a tank? I want him to deal damage while being able to soak up the damage with the emphasis on the latter.

3. At what level do you recommend dealing with Raedric? I have had my head handed to me with a party of 6 at level 4 no matter what tactics I've tried.

4. Enjoying the stronghold but I get this "20% experience bonus for 4 turns." I assigned my one additional party member to it. And there it sits. What does this mean?

Thanks.

For Raedric et al, hang behind a bottleneck and have one party member kite them into a kill zone in front of the bottleneck. Be prepared to heal your lure.

There is a couple of ways to beat Raedric that work well.

When I played it there was a bug in the game so wouldn't let complete this quest and I fought him many times over.

1.
Should be a relatively simple process:

Edit grimoire then remove spells you don't want to use by left clicking on them (on the right side…)

Then choose your new spells from each level selection on the left. If you find additional grimoires, simply edit them and buy any unknown spells (they'll have a + symbol) for Aloth that way.
I have had two mages in my current game and so have got used to sharing the wisdom.

2. In the early period of the game for fighters I've stuck mainly to medium armors and haven't really gone for the plate/heavier stuff. You can enchant them if you've the ingredients to add stats/resistances to specific damage types. For damage soaking with your tank, be sure to check out the defensive talents, there's some great options for that.

3. I'd try again at say level 6, depending on what difficulty you're playing on.
It was a challenging encounter even for me on normal the first time; I did it around level 5. There's plenty of areas to explore in the meantime if it's too difficult. (Endless Paths for example…)

Curiously, in my current game, my group chose to side with Raedric. I was hoping for a little more reactivity to reflect that decision (particularly compared to the more obvious choice in siding with Kolsc) but haven't really observed too many changes. Perhaps this will only become really apparent in the ending sliders after finishing up.

4. Not sure on this one - sounds like you've assigned a task to one of your companions whilst in the Stronghold. It'll be a quest of some kind, as I don't think there's a resting reward with those particular statistics.

In any case, enjoy the game.