Hi Naveen, I don't how if your ELISA will work in 2.5M guanidine. I am guessing that it wont work.
Have you tried to get your protein into solution by reducing and alkylating it? The protein will probably be of no use to you it is a precipitated tangle. You could also try using a buffer with 20% ethylene glycol after reducing/alkylating.
If serial dialysis is not working you can change buffer directly, to avoid precipitation you can use DTT or other reducing agents. If you are not able to do this the best way is to go as Miral Ladani has suggested.
Protein work is very cumbersome and frustrating, you need to do it very carefully and with patience.