Re: Venomancer lvl 7 roshan
I wouldn't bother using Plague Wards until I am at least Level 5/they are Rank 3. Plague Ward is a skill that starts weak but scales dramatically with investment (+125 HP/+10 damage per) so it's best to play with that in mind or you may find the enemy mid feeding off them while you gain nothing from their use. As such, I get Venomous Gale and Poison Sting at Levels 1 and 2, then move on to a skill priority of Plague Wards > Poison Nova > Venomous Gale > Poison Sting. Poison Nova can be delayed until Levels 10, 11 and 16 if you aren't building Arcane Boots or Aghanim's Scepter.
There are three main issues with solo'ing Roshan as Venomancer; it's slow, your wards won't tank for you if you are in the pit and it's nearly impossible to pull off if you aren't on Scourge (Dire in Dota 2) side. If you still want to solo Roshan as Venomancer, the best way to overcome said obstacles is to first stack 7-8 Plague Wards inside the Roshan pit without aggro'ing him. Then, when you are ready to begin, you simply aggro him and kite him around his pit while planting another Plague Ward everytime it comes off of cooldown. If you can get an ally to tank Roshan for you, all the better.
As a carry hero, Venomancer is below average; +2.6 AGI growth, debatable farming ability (Plague Ward) and no real steroid. If you must carry with Venomancer, you'll need to get a good start (farm and/or hero kills) for sure. Carry Venomancer could easily be built like Drow Ranger; Power Treads, optional Lothar's Edge (Shadow Blade in Dota 2), Helm of the Dominator (stacks with Poison Sting), Manta Style, Butterfly, etc.