Most ready to solder tin sold has a flux core. Meaning you don’t really need flux, that is, if you do it the way the original commenter did said. 

Another thing that can help is a soldering iron that doesn’t take 15 minutes to heat up. If it takes 15 minutes to heat up, there is no way it can transfer enough heat for more than a few pins, as the iron will cool down too much to get a decent joint.