I find using some flux paste really helps to get the solder flowing, I got a big tub of it in the plumbing section of B&Q (yellow tub) - think it was about a tenner but should last for ages - I think you're supposed to wipe off the excess flux from the joint afterwards to stop corrosion.
Also make sure you use quite a broad tip as the terminals/wire used are very good at taking the heat away from where you want it - the broad tip will be able to resist this better and deliver the heat to the joint.
I think I'm just using some bog standard cheapo ebay solder with an Iroda gas iron (broad tip) and the flux and it seems to get the job done fine (battery terminals, motor connections etc.)
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