I've got a wizard with 11 Dex and I'm trying to figure out what my most efficient use of spell slots is to help me from getting hit.
We typically have 2-4 very difficult encounters per day and I want to balance my spell slot uses where I can provide as much firepower as needed. The monsters are generally higher CR as opposed to mobs. It's usually approximately 1:1 in terms of PCs:NPCs and if it isn't, it's fairly close. Mob fights have been much less common.
Spending slots on defense with potentially diminishing returns due to the actual AC values is where I am looking to be most efficient.
Currently 5th level and have the following spells available:
1st level - mage armor and shield
2nd level - mirror image
Scenario A is casting mage armor in the morning and having AC 13 throughout the day. The difference between 10 and 13 seems minimal and that i'm likely going to get hit no matter what. But casting shield will boost to 18 which is legitimate. But then I've burned 2 spell slots on this method.
Scenario B is casting mirror image in a fight I think I'll need it. Leave my AC at 10 and hope the images take the hits. Can still use shield if AC 15 will save me at some point.
I'm open to other scenarios as well, just trying to figure out the optimal strategy to get the best return on my spell slot usage for defense vs offense.
My offensive strategy
Most of my offensive spells are evocation and that's where I'm mostly going to spending my spell slots. I'll have a couple of control type spells that aren't, but a lot of the instant-damage I do will be evocation.
While I try to stay away from enemies and not be in melee range, it's going to happen at times. It's those times (and ranged attacks), that I'm most concerned about keeping myself alive after.