Please specify whether you want to isolate any specific type of collagen (e.g. acid soluble/pepsin soluble/insoluble). If you want to measure the levels of different forms of collagen, you can use different kits which are commercially available (sircol dye based, hydroxyproline based). You can measure the total collagen of your sample as well.

I agree with Rajib. An ELISA to measure collagen is perhaps overkill. I know you specifically want to measure collagen II, but if you are working only with cartilage tissue, you can assume that you have type II collagen and chondroitin sulfate. Therefore, I would use a sircol assay. To homogenize the tissue sample, you can use the manufacturer's protocol. If you are still having problems with homogenization, try a ultrasonification step...1 minute is usually enough to loosen cartilage so that the homogenization step woeks efficiently. 

I'm feeling contentious this morning so I'm going to say that its just not possible by ELISA. The very nature of collagen being cross-linked means that it is very difficult to solubilize completely, even after pepsinisation and the only method I know that will allow accurate quantification of individual collagen types within a tissue is cyanogen bromide digestion. This does digest whole adult cartilage completely but the analysis is complex and the method potentially hazardous. Assuming you are working with adult articular cartilage, the proportions of type II to the other collagens present in the tissue are fairly well documented in the literature so I would concur with Rajib and use something like the hydroxyproline assay to measure total collagen in your sample and estimate the amount of type II. Any form of homogenization will mostly result in small bits of cartilage and not soluble collagen so acid or pepsin extraction is required depending on what you want to do with the collagen.