As a few have said, the most obvious way is that SE gives you the skills to move in and out of melee range (as shown in the trailer) and its up to you to make sure you don't stand in the fire. Don't ever jump to the front of the boss for example. Squishyness should only matter in these cases for unavoidable damage, which if you aren't screwing up in other areas shouldn't be a big problem.

On people who think that going into melee is going to be a rare thing: The placement in the trailer makes it seem that jumping into melee and then back out is going to be a big part of getting the big spell off. I wouldn't expect it to be rarely used or that SE will suddenly change RDM to only ranged.