Your specs ask for 200W from a 6V supply. Thus it needs to provide Ampere levels upwards of 30A. It won't be easy to do this. Parasitic resistance needs to be so low that it is an unrealistic expectation.
Can you connect a clip to the middle of your battery stack? Then you could charge it in two sessions which is an easier task for your 6V supply. Or divide the stack into 3 sessions, or 5 sessions, etc.
It is important that you are able to locate a bad cell, or a bad connection. This is easier to do when you are charging fewer cells.