Certainly not an expert on Deans Plugs but over the time I ran RSM Distribution and sold electric power systems I did purchase and solder up over 5,000 Deans Plugs. What I used was a 40 watt Weller Solder Station with a 1/4" chisel blade solder tip. Station works very well. Just tin the connection blades and apply the solder tip to the back side of the blade. I used 1/32 flux core solder purchased from Hisco (63/37032PSENG+1 SOLDER 1# SPOOL part # 110170) a bit expensive but superior, clean, smooth solder joints.
Purchased the Deans Plugs (copies) from Hobby King in bulk because they were high quality nylon with "grips" so they were easier to pull apart than the hard plastic smooth Deans brand. Always plugged a male plug into a female plug to keep the connecter blades aligned.