Use credits.
I spent over 200$ on packs so far and the only decent thing I got is a crystal that sells around 300k on GTN.
The packs are a scam.
If you want to make credits fast, you can buy pack and then sell it on GTN for about 300k.
If you don't want to grind credits in game and do not mind paying a little, you can make a fortune selling packs.
Let me know if you have more questions.