I'm not sure I understand the problem. I have soldered EC5's to CC 120HV ESCs before with no issue. I usually have to either snip a few strands of the esc wire off to get it to fit into the connector, or I get the tinned end of the the wire heated up a bit and use a pair of flat grip pliers to crush the end to form it so it will fit into the connector for soldering. Is that the issue you're talking about, or something else? If something else, can you take a picture and post it to give us a clear idea of what the problem is?