Personally I like to use Fire Wall when you're overwhelmed with enemies, it helps hold them back a little easier and it also deals some damage to them while they try to cross it! It also depends on what kind of mage you want to be though, if you're focusing solely on being an elemental mage, then it depends what element you're focusing on (No duh Ruder, of course that's true ;-;)
You'd be able to set a fire wall anywhere as long as it's on the ground and you've got enough mana. When attacking what I like to do is set a fire wall, lure an enemy into it, and then strike them while they're burning or set one right on top of the enemy. I also like to stand in/next to the fire wall since it only does harm to your enemy so that's good I think, it serves like a barrier for yourself (sort of, enemies can still technically hit you but they burn while attacking) and a weapon for your enemies.
The cons about fire wall is that it takes a lot of mana (I think) so it's kind of costly. I don't know how long it lasts, but I think it's shorter than those lotus lizard barriers (ahaha me and my little to no knowledge about Paw'd, a game I've been playing for more than a month now).
Not sure about any other advantages and disadvantages, there's someone out there who would know more than me for sure. I hope I helped though, even if just a little.