With large wire, you need a lot of heat. I do tin the plug but not the wire, it may prevent the wire going into the plug. Try to heat the wire and plug at the same time.
If you’re using EC5, you will solder the connecter before you attach the housing. Be careful not to get any solder on the outside of the plug. If you’re using another connector, attach an extra connecter, plug it in to the one you are soldering. This will prevent damage, holding the plugs in place if the housing melts a little.
I use a propane torch for ec5 connectors. Tin the wire first, put some solder in the ec5 end and melt it, shove wire in while still over the torch(holding both with pliers!). Some solder should spill out. I never wear any protective gear, but I suggest a face shield and gloves. Very least look at it sideways and dont get your face too close. Most of my planes and batteries are ec5. Soldered a hundred of them that way, have not had a bad solder connection yet. Using an iron you can not get hot enough to have a reliable connection, the joint will run a high risk of failure.