Well, it has been published as a playtest, so it could (and in my view should) get a buff - preferably a boost to the EoT effect.
Actually I think any defender would work. Place the sphere two squares from a Paladin that is marking an enemy to keep the enemy away. Fighters (including knights) are the optimum, but most defenders should allow some nasty "gotchas". Remember, too, that you can't go through the actual sphere square (it "occupies" its square). Edit: putting the sphere just covering where the enemy could charge to seems to work a treat, too. Charging has to be on an "always closer" basis and ends your turn, so it's quite possible to make FS really inconvenient, there.
On the "you lose the initial attack if you place the sphere to block" issue, just use a standard action to conjure the sphere and damage with it (as usual) and then a move to place it in the blocking position. Nothing says you can't move it on the same turn as you conjure it.
The original FS gave you more mileage out of the spell because you could attack with it and then move it over to another group of enemies and they take damage when their turns came up. They would then move because of the sphere and because they were probably going to move anyway.
The main problem is you really don't know when a creature is moving because he has to or because he was going to anyway. Mostly from my experience they would move because they didn't want to get burned again and they were moving anyway. With EOT to the FS the creatures aren't taking damage and thus the whole point of the spell isn't working.
Damage spells should not be EOT because it pretty much makes the spell useless. If you aren't damaging enemies then there is no point to the spell.