I believe mine is 40W - KSGER with STM32 controller and T12 soldering tips. Watts don't matter that much. As many have said - it is a lot about thermal management.

1. choose the right soldiering tip for a job (I use T12-C4 for connectors this large)
2. plug in the opposite side of the connector, maybe even with some wires on it - this will pull heat away from the soldering end of the connector pin. Keeps the rest of it cooler and prevents plastic melting.
3. Fix the connector and a wire with a vice, clamps or alligator clips (lookup "soldering helping hand" on Amazon). When I do the job I have a strand of flux core led in one hand and a soldering iron pen in another. Wire and connector are fixed in a rig and touching each other
4. Flux the ends of the connector pin and a wire. When doing battery wires I like to use liquid flux instead of gel or pen. It soaks all the strands inside and that ensures that it will suck in the melted solder.
5. Before touching anything with hot soldering tip - make it is "wet" first. It must have a nice melted bulb of solder with steaming flux on it. And only then you touch the parts to be soldered with that bulb. It ensures effective heat transfer. The goal here is to increase the contact area between the soldering tip and whatever parts you are trying to solder together (connector pin and a wire).
6. Keep a soldering flux core wire in the other hand to push it into the soldering flux contact in sufficient amount.