Three casts of Banish Undead does 1755 damage. A cast of Scourge does 544 damage, and a cast of Venom of the Snake does 453 damage. So one Scourge and three Venoms will do a total of 1905 damage, and cost 650 mana. That's 25 less mana than your three Banish Undead costs, and considerably more damage (granted, the monster also regenerates HP, but very slowly).
Now, let's think about the simplest scenario. Forget about cannibalize, forget about regen, forget about the pet, forget about buffs. Just casting those DoTs and rooting, a shaman is already a better soloer than a cleric.
Then, we can think about adding canni+regen, which increases your mana regen overall. You can say that regen requires upkeep, but this is not true because it pays for itself. Chloroplast at maximum duration (level 60) heals about 2000 HP for 200 mana, and at minimum duration (level 39) heals about 1300 HP for 200 mana. Canni I gives about 25 mana for 50 HP. So if you cast Chloroplast, you have to spend 400 HP to regain the mana for it, and the rest is free.
Then, we can think about adding the pet. Pets regenerate HP on their own pretty quickly. You can send the pet at the monster, let it do some damage, then back it off and let it regenerate on its own (no buffs, no nothing). This is free damage. You never have to recast the pet if you don't get it killed. If you decide to buff or heal the pet, this might make it better, or it might not be worth it, but if you JUST use the pet at a baseline, it's straight-up free damage.
So the shaman starts out with root+dots already better than the cleric. Then you add canni+regen and he's better still. Then you add a pet with no buffs or heals and he's better still. Then you can think about adding other buffs, or what have you, and improve him further. There's simply no question that the shaman is the better soloer.
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