[Tallenn]

At 8, you can still solo effectively, but grouping will be more fun (plus you have to go all the way to Highpass to get your pet). If in FP area, orc camps in EC, if in GFay, go to Crushbone.

For quests, once you hit 12, you can do Stein of Moggok (DE illusion to do the final turn-in to Clurg), unless you are a DE, in which case you can do it at 8. It's a LOT of running, but it's the best off-hand you will get for many, many levels, and it still sells quite well.

I haven't really done much to time mezzes- hopefully group is killing fast enough to get to them before it does in most cases. Otherwise, I usually find out mez is of when I start getting hit, lol.

There are no diminishing returns. I think other games started adding stuff like that because they felt EQ enchanters were too powerful at CC, making it tough to build challenging content via multiple mobs.