Would be helpful if you could provide the following details :
1) What is your max MP?
2) How much remaining MP do you have when you enter phase 2/3
3) How many people stacking for Repelling cannon? (Pets do count IIRC)

The hard hitting attack "Resonance" does not have a cast bar, but the usage is still scripted, in Phase 1 is as below:
1) Directly After Nerve Gas
2) After the Stack/Spread Mechanic (Boss will auto 2~3 times before Resonance)
3) After Secondary Head Mechanic (Boss will auto 1~2 times before Resonance)

At the very beginning, I think there's a double Resonance that may happen depending on your dps, but kinda foggy about it =\