How to approach designing a 30GHz bandpass filter?
For 30ghz, there likely are no commonly used active filter topologies.  However you can use galium arsenide FETs in this region to build various circuits (PA, LNA, mixer, VCO, etc), never seen them used at 30ghz but have seen them used for 24 Ghz I would think you could extend their operation to 30Ghz if care was taken.  

Passive filters will definitely be implemented with microstrip structures, lumped elements will not be useful at these frequencies.  You'll also need to find a very good substrate to work at 30Ghz.    

Falloff is dependent on the topology you use and the number of poles in the filter much as it would be at lower frequencies.  For instance Chebyshev filters are common.

My only experience near such frequencies was a 24Ghz FMCW radar front end, that used GaAs FETS for the PA, VCO, mixer, a hairpin bandpass filter and was built on Roger 5870 substrate.  I didn't do the RF design this was an externally sourced design and we ended up using a 10.5Ghz variant of it.  

My initial thought is that your desired bandwidth is really wide, and that will greatly complicate the design.

Your best bet is to get a couple books on RF filter design, make sure your up on your calculus and start running some simulations using various topologies.  Ansoft Designer and Microwave Office are the two simulation packages that i've bumped into in the past for RF circuit simulation.  I believe they both have free trials.