How best can I use Raspberry pi and ASIC?
Let's do the math. The average time to find a block, which is worth a reward of at least 25 BTCs (+fees) is the following:

avgtime = difficulty * 2^32 / hashrate


The current difficulty is 16,818,461,371. Assuming a hash rate of 2GH/s we get in hours:

avgtime = 16818461371 * 2^32 / (2*10^9) / 60 / 60


That's about 10032602 hours or 418025 days. To mine solo successfully, you need much, much more hashing power.