Hand-soldering stripline opposed emitter transistors
There is a book called Radio Frequency Transistors, Second Edition: Principles and Practical Applications (ISBN 0750672811) that has a section on mounting and soldering SOE RF power transistors. This is a summary of the most important points made in the book:


Use a relatively thin layer of thermal paste on the heatsink area of the transistor to prevent the flange bowing up.
Physically mount the transistor with a flat washer on top of the package and a split lock washer on the other side of the PCB or heatsink. 
Be careful not too apply to much to torque to the fasteners because it stops the flange from expanding with heat. This ends up giving worse thermal contact than not enough torque.
Try to keep the leads as close as possible to the PCB tracks to minimise inductance.
You should fasten the transistor in place before soldering.
When soldering use apply a generous amount of solder to leech most of the gold from the leads to help prevent a brittle joint.


I've only ever soldered a couple of these devices but a technique that worked well for me was a reasonably large chisel tip and 1mm solder swiped outwards while applying a light pressure on top with a pair of pliers in the other hand. Then I just quickly worked back over the top of the pin with more solder right along top to keep the solder molten the whole time with a delay in between each pin. 

That was with a package with long flexible leads that sat flush so your mileage may vary with some other packages. Doing a Google image search on MRF454 will show a few techniques, some appear to use the method I've described while others appear to just concentrate on soldering the ends of the leads. It will of course also depend on your PCB and whether you have copper underneath the full length of the pins.