Generally, hit them hard, hit them fast, and then bounce away, heal, and do it again. You probably can't out sustain them, so breaking it up into two or three encounters is likely necessary. Kill the thought-forged first, usually, if it's there, unless you're bloody sure you can kill the solipsist before it gets to you; while the thought-forged critters are up (and beyond the critters themselves being pretty dangerous) the solipsist gets some pretty major bonuses (Mindpower/resall/mind speed from warrior/defender/bowman, respectively.). You almost never want to engage at range, if you can avoid it.
Sleep, unfortunately, is a bit of a special concern. I'm still not entirely sure how to deal with it successfully beyond an already running providence or high mind save. Depending on the solipsists talents, it's basically an improved daze only instead of breaking on any damage, it only breaks after a certain threshold. Given that enemy solips can break the talent level limitation, it can get seriously nasty.
As for the specific questions. They move away because of the tactical AI designating them ranged attackers; they don't want to be in melee with you, ever, and will generally step away in all situations, if you're beside them and they can.
The second question is because of the eponymous solipsism talent. What that does is convert a set percentage (Most enemy solipsists are probably going to have it at 100%; human solips need a category point and 5/5 in the talent to hit that.) of all incoming damage
and all healing into psi damage -- that is, the damage is taken out of their psi resource instead of their health. This basically converts their psi bar into their health bar, which is why they seem to be so incredibly tanky.
There's a couple of downsides to that. The primary one is that it means you basically eat into their offensive capability at the same time you eat into their effective health. The more damage you do to them, the less psi they have to power their abilities. This is offset to a decent degree by the fact that their psi bars are generally pretty huge, because the solipsist tree talents all (and cumulatively) reduce the HP gain from con and boost the psi gain from willpower. The secondary downside is that once they're below a certain threshold (50% is the important one) they start taking penalties. A full psi solipsist is going to have at least 150% global speed, ferex, while one with 25% psi is going to have like 75% global (-25% global speed malus), and the precise global boost or penalty is going to fluctuate with the psi bar. There's another penalty involved that I forget the specifics of, yeah.
So this means that the more you hurt them (and more specifically, their psi bar), the worse they are, but when you first encounter them they're at their absolute strongest. This is one of the reasons they're particularly nasty, I guess. But yeah, the more damage you do, the worse they get, so, as stated, hit hard, hit fast. Try to knock off as much of their psi as you can, so you can start getting their penalties rolling and block off some of their nastier stuff. If you can get their psi low enough, they become pretty helpless. It's just the getting there
