Frost Flare is best used for sniping. It has relatively low damage, but compensates with a rather serious combat art range. Zoom out as far as you can, advance carefully and start sniping the enemies from far away. The reduction in movement and the damage over time stacks as long as the effect from previous casts still lasts (refer to the CA for duration time on that matter). Be careful with enemy groups where hitting one mob will turn all of them aggressive towards you. In those cases use a crowd control combat art instead.
The flat ice damage can cause an additional slowing effect by triggering the Freeze secondary damage effect. This stacks with the built-in slow effect from Frost Flare and can slow enemies to a crawl, and is enhanced by Damage Lore.
Pros and Cons
Decent damage over time
Very long combat art range
Great for fast sniping
Does not aggro surrounding mobs as long as they are not a group
Reduction in movement speed increases with consecutive casts on the same enemy
The Damage over Time also stacks with consecutive casts and does not overwrite the DoT from the previous cast on the same enemy (as long as the previous DoT effect lasts)
Relatively low damage
Does not work against groups of enemies (without "Icy Circle" mod)
Hard to balance between damage and regeneration time
The game has fixed value for maximum DoT duration. It is defined in balance.txt by DurationDOT parameter and has default value of 500 (5 seconds). So even if Frost Flare has its duration listed as 10 seconds, it would only deal DoT damage over 5 of them.
If Constraint modification is chosen, Frost Flare would reduce target's attack value for the time listed as spell Duration in its description (same as the duration of movement reduction; initial value is 10 seconds). That makes Frost Flare more effective for attack reduction than Raging Nimbus, which decreases attack only for one second unless the target stays in the Area of Effect. Reducing attack value modifications for HE's CAs "Frost Flare" and "Raging Nimbus" are added with each other multiplicatively (layerwise). For example, 120 CA level "Frost Flare" and 105 CA level "Raging Nimbus", used on the same enemy, will reduce target attack value by 66% at first and by 61% secondly. The final target attack value = x*0.34*0.39 = 0.13*x Due to the peculiarities of a chance to hit calculation this means, that HE will be hit by enemies 3-4 times less. To effect stacking you must use CAs with a difference of not less than 0.1 CA level.
"Icy Circle" gold modification, increasing the radius 3.3 times, increases the area of effect in 11 times, because it depends on the squared radius.