Yea, when you kill despor 5 times, you can boost it, and as said, you can kill in the order you want. The more rounds you pass, the bosses become stronger. I recomend you kill Bruton and the ghastly one first. Bruton take 0% physical damage, so for ek and rp are going to be hard to beat if you don't kill him first and same with Greedok (ghastly) he take -50% from death.
The rest are very easy, just SD and take care with fire/energy/poison bombs, some of them heal with this elements and they throw this bombs (the one looks like a dragon lord heals if you attack it with fire, green dragon with poison, the frost dragon with ice, and Vilear with energy)