Not every spell can be useful in all situations. I know I modified my tactics when dealing with targets that moved more than others while using focused 710. I never really ran into what you did where even moving into the room has it dissipate. If I could move in the room it generally stayed. But if the target moved a lot I would modify and do one of the following.

710 + 410 = Yeah they get back up but gives you time. (I did this a lot when using 3-4 710s to kill targets in a swarm.)
708 + 710 = If you are good at spell aiming, target a leg and take it out. Then 710 just builds on top of them.
710 + 706 = More time to build.

I know these are a bit mana intensive, but they tend to work. If you still have a mana problem then maybe that isn't the way to hunt. I'm also just offering suggestions, not saying you are doing anything wrong. I personally have never had much trouble using focused 710, but results may vary I guess.

I'll admit 710 is mostly a deal with you while I'm worried about something else to me. I'll cast 710 on the second, third, and fourth target in a room. (Of creatures that don't move much.) And use other tactics to take out the first while it spins up. I don't use it much as a standard hunting mechanic. I can see 709 working well too.