New plan: give up on killing Imp until I'm geared enough for H Imp PUGs.
Also going to keep up with my Mage since I found out Fire doesn't use Prismatic Crystal.
Just keep at it, he's a very doable boss. For PUGing it's all about getting the right people, and, if you end up doing a number of attempts, keeping overall group morale up.
We got our H Imp kill last night, and from all the troubles we had with PUG'ed people, my best advice is to start your own group for him and enforce a one-strike-you're-out rule. If someone blows a mine once or twice, they're probably going to do it again, and in my experience it's better to cut your losses sooner rather than later.
A lot of people don't want to hang around in a group if they feel the group leader is being soft on people continually preventing the kill (some are dicks about it and some aren't), and you can keep them confident by trimming the fat. It's kind of a harsh stance to take, but when the fight takes 7-8 minutes to get to the point most people wipe at, a lot of the group's time can be wasted by trying to carry accident-prone players.
I'm a pretty lenient RL, but Imp is where I have to buckle down. He simply eats up too much time. If it's the same small handful of people that are consistently the weak links of the group, it's wasting the time of the other 15-20 players in the raid. It's not a terribly big deal when the wipes happen 1-2 minutes into the fight, as can be the case on the other bosses in HM, but when we're wiping at the ~10 minute mark, I need to be respectful of the raid's time as a whole.