The higher end mobs aren't the problem. Since you don't have to kill that many of them, you can take longer to kill them.
The problem comes with the second and third tier monsters who sometimes have a lot of hit points yet don't hit very hard. If you aren't killing them in 2-3 hits, you'll never advance the spawn by yourself. For this reason, you really need an appropriate slayer weapon for the spawn.
An example of a solo-spawn, Mephitis:
Level 1 (spiders and scorpions): rush in, pull as many of your opponents as you can at one time, whirlwind them to death. Cure yourself after each crowd is handled.
Level 2 (Drones and Warriors): Switch to a terathan or arachnid slayer with fast swing speed. Use the same tactic as before, but if you don't have a whirlwind weapon, then use momentum strike for groups. Without a slayer weapon, the warriors can take too long to kill to advance the spawn.
Level 3 (Matriarchs and dread spiders): This is where you'll get stuck. If you rush in, you'll get pelted by 100 spells at once and die. Getting fireballed/magic arrowed to death sucks. If you don't have a weapon that can kill a matriarch or dread spider quickly, you'll never advance past this level. Stay at the edges and work your way to the middle. If you get to the altar, head back to the edges and work your way around, since the spawn may advance soon afterward. You don't want to be in the middle of the spawn when it advances. If you think you've been there too long, head for the middle and check the candle progression. Try to keep the battles to you against 2-4 opponents.
Level 4 (Avengers and poison elementals): You'll have to clean up leftover dread spiders and matriarchs to clear a path to the altar, but then you can take your time working your way to the altar. 9 times in 10, when you're soloing a spawn, Mephitis is up before you get all the way to the altar. Ignore her uness she targets you while you clear the spawn. Alternatively, when you know she spawned, you can honor yourself and pull her out.
Mephitis: Once she's up, it's just honor her, EoO, and kill her. Use your best weapon, even if it's not a slayer, between EoO, item damage increase and perfection, you'll hit max damage in a few hits anyway.
Most spawns have a similar progression. I haven't soloed Semidar (demons) or Rikktor (reptiles) because of the difficulty in killing multiple succubi or Rikktor himself, but the others (Baracoon, Mephitis, Neira, Oaks) can be soloed by the well equipped sampire.
If you try it in Ilshenar, paragons will seriously ruin your day. It takes far too long to kill them and the level 2-3 paragons will almost always slow you to the point where you lose candles unless you have the perfect slayer weapon for them.
