So I started a spellsword type character, and was trying to decide between Conjuration or Illusion as my defensive school of magic, conjuration would be to summon aid when in great need, or Illusion to help deal with groups.

What I noticed was that certain enemies were not affected by Fear/Frenzy/Calm as they were "too powerful" my question is, how effective can illusion be once you get the appropriate perks, how can I tell when enemies have grown to powerful for my magic? Also, if I activate those perks, will it reflect on the tooltip for the spell? will frenzy update from affecting enemies lvl 9 to whatever the perk allows? Thanks for any info you can give!

Once you get all the perks in the tree you can control almost anything in the game even the Dwemer Centurions and high level undead.

Dual casting increases the level of the monsters you can affect as well.

yeah illusion seems to be the only magic skill that scaled properly and has useful perks IMO.

I'm level 51 with illusion of 95 and most perks in the tree. Every run if thee mill mob is affected by my spells. Some more special mobs like the Thalmor mages require a dual casted spell. Few are immune altogether, like Dragons and some boss types and named enemies.

Even when I find am enemy it doesn't work on, like a boss, I can affect all of his friends. Either pacify them or frenzy them to make them attack each other or thee boss.
Also note you get upgraded versions of the entry level spells that affect higher enemies and hit with an aoe to affect more then one.

Lastly, in dungeons fear/rout is a terrible spell. All your enemy does is run deep into the dungeon scared and alert everyone on the way. That's why I didn't bother taking the second perk that boosts fear spells.

I still wish we had what we lost within the illusion spell group.

Enemy levels stop at 50.
A dual cast pacify affects creatures and people up to level 20.
With Hypnotic Gaze perk it affects up to level 28
When dual cast that doubles to 40, or 56 with Hypnotic Gaze.

Against men/mer and animals, it'll affect even higher so you won't have to dual cast it vs. them as often.

Dual cast doubles after other additions are made to the level the spell can effect. A dual cast pacify, with hypnotic gaze, and master of the mind, can calm anything other than dragons which are simply immune no matter what. My character can calm Dragon Priests.

IMO these are the perks worth getting in Illusion -
http://chrizel.github.com/skyrim/#t/1/3ybif4,0,0,0,0,0,0,0

Details on how it works here-
http://www.uesp.net/wiki/Skyrim:Illusion

yeah I don't like frenzy in skyrim I liked it in oblivion better but i just wish some of the other magic skills could have better perks and scale well like illusion

I wish we could create our own area of effect and magnitude of the spell ourselves.

The frenzy I skyrim doesn't work very well. Enemies still usually target me at first when frenzied, which is broken.