If you’re specced for Double Up they are a source of slag. They tend to focus on one enemy at a time in my experience, but it certainly helps. I do like to have options for cool down times though. I use slag nades, the aforementioned Magic Missiles or bouncing Betty do pretty well, and all of the other options @RavenOfArisia listed are good too. With Expertise specced Axton’s swap speed is good, so considering a slag and swap style for down time might not be too taxing.
Also, you can reply to multiple people at once by writing their tags in the reply. Here’s a FAQ with some foruming tips and pointers that I wish I had read when I started posting.