If you're wanting to duo and don't want to include a shaman in the mix, the only other obvious class to add a lot of power to the duo would be a Necro in my opinion. Gives you your snare and a load of ability to deal with non summoning mobs.
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my main was a necro.
the only thing a necro is good for is aggro kiting in out door zones. the dots take time to work, and some, like the darkness and pyro series generate a ton of hate. they CAN be used as a healer up untill 50ish i would say, and quite effectively, but after that, unless you can replace 3000 to 4000 hitpoints quickly, you just cant keep up with the damage output from mobs at higher levels.
necros are masters of.....not dieing. thats their claim to fame. feign death, fear, snare give them much more surviveability than any other class in the game. but soloing or double boxing? only if your aggro kiting in out door zones at higher levels, or fear kiting in pre level 50 zones. pretty much everything after that is immune to fear. yep, right after you ding level 57 and get your shiny new fear spell, trepedation, just about everything level 60 and up is immune to fear. EQ has always been the masters of delivering something just this side of useless. wait till you get dire charm, and find there is maby one dungon you can use it in, and your pet will outdamage it because you can only charm mobs significantly lower than you will be fighting. that 115 a pop bonie you charmed isnt going to hit anything 10 to 15 levels higher than it.
not dieing is why they are so popular. that and because they CAN solo all the way to the top aggro kiting. past 60, if you dont have a healer AND slow, no class that gets hit can solo anymore.
at lower levels, 50 and below, anything will work. 60 and up, you need slow and heal unless you want to find the 5 or 6 sweet spots to sit for the rest of your characters life in an outdoor zone where adds are controllable and you can aggro kite.
anything will work pre 50. 60 and up though, your probably going to have to decide weather to start raiding for gear that will let you contunue without being one shoted by a common yard trash mob, or you MUST HAVE slow and a good healer. a mob quadding for 500 to 600 a pop at 60 will shred a warrior with the best bazzar gear really quick. faster than you will be able to get a CH off.
after 60, you start another toon, start another game, or start raiding. anything else is just soo painfully slow and tedious that only the most sever casses of Obsessive Compulsive Disorder types can bare to keep playing.