Zaltanna is Balance, so this should give you a little advantage. With her two spider storm minions, I can see why there might be a challenge, however, don't be deterred by this. Quite a majority of the bosses in this world or instances for that matter request that you face three opponents at once. Something you should get used to from now.
Now, on to your battle.
Go First - Restart the battle until you go first, gives you a much needed advantage.
Gear - How's your gear looking? Do you still have your Waterworks set? It helps. A great wand is good too!
Deck - You don't need a big deck with loads of spells. My advice to you is to have a deck which allows you to offend and defend should need be. (More on that below).
Crowns Player? - Henchmen makes this battle a lot easier!
Team Up - Tried Teaming Up?
Pet - How's your pet? A decent healing and defending pet is highly critical in attempting to solo this world.
As it relates to your battle. Your deck should only have a few blades, enchants, feint (if you've it) and a couple hits with forest lord of course being one of them. Place a couple fairies/satyrs if you happened to be criticaled. The spiders won't use their rank 8 spells as much, but rather tempest, kracken etc. However, that doesn't mean you shouldn't expect it if they've the pips to use it.
The idea here is you want to be able to defend (shields from side deck if needed) and blade while you prepare to use FL (Forest Lord). A Solid hit should damage the boss and take the spiders out.
After defeating the spiders, chances are, you may be low on health, heal, you're for a reason. Now, focus your energy on the boss. She's not much of a threat by herself honestly. Her health is low and can be quickly dispatched with a few seraphs and centaurs.
for a reason.