Do we need a legit way to effectivly farm credits?

It's 883 credits per crystal if you buy the 595k pack. Meanwhile, a refill for 100 credits will give you 120 energy. So, if we use three raids for 150 energy, simply take the total amount of credits, multiply by 0.8, then divide by your refill cost (in this case 100) to get your credits per crystal conversion. So, assuming a 100k credit payout per raid, 300k would be the total. 300k x 0.8 = 240k ÷ 100 = 2400 credits/ crystal. Even if you did a refill for 200, you're still getting 1200 credits/crystal, which is 150% better than the purchase "deal."

Conclusion, never buy the 595 crystal pack, unless you're a whale who needs a few million credits right now