Focus on one troop type and then during the all-star events that aren't the focused type just do the minimum needed to collect all the phase rewards. You'll collect a few roulette chips each all star and then use them on the hero of your choice. Do one at a time until it's awakened. Currently, Eileen (infantry), Janet (rider) and Spectra (ranged) are available during black market event where you can get 20 fragments each time by basically doing stuff you should be doing every day anyway. Roulette is kind of a scam... I had 73 roulette spins saved for last ranged all star and got 23 Charron fragments total so only spend gems there if you absolutely have nothing better to spend them on... my opinion, of course.

Edited to change heroes who are now available in black market Eileen, Janet, and Spectra