I play a lot of these freerolls on 888 that have the Hyper-Turbo 3min. blind Structure.
I feel the best way to approach these games is to play tight at the start and try to make a few chips, avoid all multiway pots when people are putting in lots of chips, if you can get in cheap with a speculative hand in a multiway pot, this can pay off well also.
After the first 20-30 mins, it will be very difficult to stay above 10-15bb and it is better to switch to a push/fold style. The players tend to be very loose, so push very tight range from earlier positions. You want to avoid multiway all in at all costs.
If you can survive long enough to get to the 5-6 digit blind levels, it is just a crap shoot. Every pot is worth a ton of chip from blinds and antes, amd one big pot during this stage can bring you straight to the top. You pretty much just have to pray for a good hand or a good spot before the blinds eat your stack up.
Use your timebank wisely, tanking in this structure can be quite beneficial as people are busting out at an extremely fast rate. Tanking for 30 seconds you could see several hundred people bust out in such a small time.
Ive made the final table a few times in these very large field freerolls, but have never captured the victory.