When trying to get into the part with all the spikes, jump then wait until you are in between the top and bottom rows of spikes to use the Mach Dash. You should use X to do the Mach Dash after jumping and release all buttons at the time so you do not hit the second row of spikes. After making past the first row, rescue the Reploid and repeat the procedure for the next row of spikes. Do the same thing to get out.