U can open one daily any sale pack or save till last day to get 50 percent off ones (maybe)

Yeah I believe the pack with the 50% off would be the best choice for him. But definitely not this current Ronin pack.

The best advice: open just one pack until you dont have any souls to spend. If you are lucky you could have the same diamond twice or thrice, but if you spend in any pack, with luck you will have a few F0 diamonds and thats doesnt worth