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...cancel/annul magic will also remove all the debuffs cast on the mob (slow, tash,...
Both Tash and Slow are actually not cancelable with cancel magic. This was changed due to that exact reasoning (debuffing a mob would cancel them). Tash adds poison counters and Slow (most of them at least) will add disease counters (This is true of slow, even if using the MR based one.) So you'd have to cure poison to remove tash, and cure disease to remove slow. This works when cast on players btw..so you don't have to dispell all your buffs (or wait) to get rid of these nasty debuffs.
This did change sometime awhile ago, they used to both me able to be magically dispelled Not sure about malo/mala lines...but I believe those add disease as well. (Could be quite wrong...I don't play a shammy/mage and I'm trying to go off by what has landed on me).
Weaken/Cripple though will be removed.
At higher levels, clerics get a nice line of "reverse damage shields". These are great as they are cast on the mob and don't take up a precious buff slot. They will do damage to the mob whenever the mob hits someone, which is also nice as the DS is concentrated rather than having to cast it on everyone who might take damage.
Overall though, unless there's mana/buff slots to spare, a DS isn't very efficient vs a slowed mob, (it's taking ~70% less damage than it normally would be...but so are you ;)). Of course, if there's room/mana to spare, every little bit helps.