So you are considering playing a controller. Everybody will tell you that controllers only really work in groups, but that is quite wrong. They excel in groups, but can function in solo mode quite well.
This little guide will not be exhaustive, just some tipps from somebody who loves the archetype.
1) Containment.
From the paragon wiki : "Controllers do extra damage to any target that is already Held, Immobilized, Slept, or Disoriented"
And that is not just "some" extra damage, in fact it's just *double* damage. Most controllers get a root and a hold in the first few powers. Those 2 powers usually do moderate damage each (often as a dot). So what you do is you slot those 2 powers for damage, accurracy and maybe recharge. With those you already have a nice attack chain with very decent damage. It's often nice to start a fight with an area effect root and then kill single mobs one at a time.
2) Confuse.
A lot of controllers get confuse powers. Some really early, some later. Confused mobs often attack their allies. When a confused mob takes half of the HPs of a target and you do the other half, you still get about 75% of the xp for that mob. So usually it's worth it to confuse some mobs and then do your damage on top.
3) Pets.
For most controllers, pets are a late game thing (with the exception of illusionists where the first pets come at level 18). Some pets are absolutely game changing with their damage output (fire comes to mind), but all controller pets are really good damage sources. So for those controllers struggling to solo at first, there is light at the end of the tunnel.
4) Resistance debuffs.
Those can come from your secondary power set. Tar patch (dark), enervating field (radiation) or freezing rain (storm) come to mind. Often people don't know that those powers are pretty nice damage boosts! This also applies to team play of course.
5) Direct damage.
Some controllers have high damage attacks in their primary set. Gravity has the best one with propel, but some others have decent attacks as well. If you want to kill single targets fast and like the style, try those. Always combine those attacks with containment for double damage.
Conclusion:
Controller solo isn't the fastest thing. But it's usually pretty safe. Most controllers kill mobs one at a time, AE damage powers are usually very weak (with some exceptions that do somewhat nice AE damage). If played well you will never die though.