I agree that defilers are in pretty bad shape right now.
Soloing is possible, but the fundamental tactic is exhaustion of an opponent, not dps burst - it's actually impossible to talk about "dps bursts" in defilers case, considering most damage comes from reflect damage, fire gourds and a couple of melee skills, of which one has a chance to stun the opponent, thus giving you time to apply the debuffs on him/heals on yourself. Flies have been nerfed to the point of uselessnes.
Speaking of debuffs, they are fairly decent, but a bit outdated, considering beast stats of nowadays. Another matter is heal intensity - if you trait for maximum heals, you sacrifice survivability (and vice versa), and i guess it's up to you to find the balance you're most comfortable with.
Defilers pre-U10 were pretty much beast-mode, but ever since the latest update came out, i'm getting shreded by insane crits from skilled players, the difference, in my experience is the most visible with burgs/champs.
P.S.: it goes without saying that Armor value, Resistance and Evade pots are essential.
Vilya: Healadder (defiler, R10), Kommisar (stalker, R7).
Snowbourn: Superserial (warleader, R8), Steelstack (reaver, R6).
Brandywine: Manvet (champion, R10 - Retired); Stargash-1 (reaver, R12)