i would stick with a shedding rake to get deep into the undercoat (i use the PSI Heavy Duty Shedding rake #809, the one on the right:
and a wire pin-brush (for rough double-coats) to finish the job.
IMO, the slicker brush you are using (green one in the middle) will only pull off the top layer and you want to go DEEP to the skin... the loop on the left gives the same effect, good on the legs and head but not on the body, rump, thighs, etc where you need to get deep down to the skin to massage, and pull out the undercoat.
I have two northern breeds (akitas) who blow their coat twice per year and trust me, without these two tools it would last 4 months instead of 4 weeks
ps: i cannot stress enough the importance of feeding a good food... please consider something better than purina, there are sooo many healthier alternatives out there! and don't forget the fish oil, too