I would use either DMPS or DMSA if you can tolerate it. Then go for the primary chelator, ALA. My personal favourite chelator is DMPS. Has a high affinity for mercury and a secondary affinity for Arsenic. Get some DMPS and start with that on it's own for a number of months, then get a good combo together of DMPS/ALA.
I prefer using two chelator's together because if the gut is impaired and you have low bile flow you can use both chelator's together to effectively remove mercury through the kidney's instead of the gut. There's a greater excretion rate when two chelator's are used, but ALA will do the job, but just take longer.