An interesting dilemma...

Can anyone tell me of a way to cast spells at a certain location without having a hidden unit casting the spells?

Example: Having the boulders surrounded by green-flames falling from the sky and turning into Infernals upon impact.

Can it be done with triggers, soley? No unit creation, or ability manipulation along with 'Issue Order' triggers?

Well, spells usually aren't literally cast per se unless an order is given. Usually mappers create a region, and then have special effects happen in it in conjunction with other triggers that make it look like a spell. Now, to do what you mentioned above, get an mpq viewer (I reccomend the BigSpecificMPQExtractor, you can get it in the Tools forum), and find the .mdx files (those are your special effects) for the dread lord's infernal spell. That's where you'll find the one that makes it look like they're raining from the sky. Then setup a trigger to create a unit after the special effect plays, like so:

Action
Create special effect infernal in region
Wait 3 seconds
Create 3 hellhounds at region

Actually I extracted the orc mission where infernals rain down from the sky there is a unit whos out of the scene and also possible invisible too casting reign of chaos (4 infernal summoning spell) in each scene.

Try opening that little sheep scenario level and look at the trigger when the character dies. There is a trigger that causes human big explotion over your character. Using a similar trigger you should be able to create the special effect of your spell wherever you want. You just have to search the mpq for the path to your special effect and paste it into a similar trigger.

-Ari

Thats a filter.