Well to put it simple it goes like this.
1st difficulty: When solo there will be NO bosses (unless its a specific story arc), just minions and lieuts. The layout is usually 1 minion + 1 lieut per mob. All enemy power levels are your current level and plus 1. (will con white/yellow to you)
2nd difficulty: The enemy power levels are the same as previous level. However now the amount of enemies in each mob has increased and you WILL get a boss in each mission regardless of whether if its an arc or not.
3rd difficulty: Now its the part of the difficulty levels to see if your character can "hack it". All enemies will be a minimum of +1 to your level. Less enemies per mob than the previous setting, however their challenge should be higher since they are now an even higher power level than before.
4th dfficulty setting: The most commonly used setting at the higher levels since it represents the most mobs you can get in a mission plus their fighting power level will be of the 2nd greatest possible too (same as before in that they will be a minimum of plus 1).
Last difficulty setting: This is the only setting where you can get an AV to spawn inside your mission while solo. All mobs will spawn purple to you (iirc) and thus is only recommended if you are truely looking for a challenge OR looking to push your build to the limit. (highly suggest doing "A heroes hero" arc on this setting if you think your build is complete - solo)
Sorry cant give much more help but its a start 
@Damz Find me on the global channel Union Chat. One of the best "chat channels" ingame!
Basically 2nd and 4th difficulty levels spawns will be as if your team size is one player larger.
On Heroic/Villainous when solo all AVs will spawn as EBs and all bosses will spawn as Lts - including named bosses
3rd difficulty spawns will be as per first difficulty but one level higher
5th difficulty spawns will be as per first difficulty but two levels higher
The difficulty required to spawn an AV varies with team size - someone else I'm sure will fill in the details.
Mind of Gaia lvl 50 Defiant's first Mind/Storm 'troller.
Deadly Doc 50 Dark/Dark Corr
and lots more on Pinnacle,Union and Defiant
http://wiki.cohtitan.com/wiki/Notoriety
Octavian Vanguard
@ohmsEU and @ohms 2
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Hey
I'm looking for some sort of guide or info that explains exactly what changes are made when, for example, you change the mission from Heroic to Unyielding, or Invincible, etc.
Also info on how many team members and what effects they have would also be nice
Anyone help me please?