I don't know if it is working properly, or if it is bugged, but acid rain is acting screwy.
First, when you cast a firewall, it goes up and the enemy takes fire damage. they can run after you, only getting hit once, but they still take it. However, when you cast acid rain, it takes about 3 seconds to go into effect, by which point most aggressive mobs have WELL gotten outside the reach. And with short duration, you have a hard time dragging them back through it.
Second, acid rain is acting more like firewall and less like melfs. By that i mean that i have had MANY times that i have tried it in the vale where when you threw it up, if the mobs only got caught in the edge of it, they would only take ONE tick of damage, rather then all the ticks the spell says it should. For example, i dropped an acid rain on some lamanaian dogs, and the ones who didnt get hit in the middle of the effect only took one acid tick. Mobs who were in the middle of the rain took 2-3 (4?) ticks as they should. In another example, the two trolls and the lion near rocky; the first troll (the one i dropped the spell on) ran towards me not getting hit at all. The other two ran twards me getting in the middle of the spell effect. I ran the first one back through the spell (getting hit in the process) just as it was ending, so it was only in the rain for about 1/2 of a second. Troll 2 and lion both took enough ticks to die (something like 230 damage with only a potency item and no acid crit item on) but troll 1 only got hit once and i had to blast it with some force missiles to take it down.
These are really bad things to happen. If a spell is going to be half as good as firewall, it cannot be shorter, smaller, harder to use on the mob you target, and harder to get it to deal full damage. It doesn't matter how much damage the spell does to an enemy, if you have to cast two spells to get the job done where you only needed 1 before.