How to get glu credits?

darioelle

To upgrade a weapon it needs to use 83 glue credits while I have just 13, the only chance to get credists, whithout buying them, it seems when I get a new level. But I earn just 1 credit and I will never reach 83 credits also because, I don't know why, my son with its tablet, can use my credits for ethernity warriors!!!
Anyway the question is how should I get the glue credits? If I just have to buy them please let me know, I'll erase the game ad I'll stop playing it...
Regards,
Dario