best combo builds?

Since throw, shockwave and incinerate only prime unshielded enemies, so you'd definitely want to kick off with overload or energy drain. The sentinel profile is good overall, but you might want the combat drone from engineer for recharge times if you're running two or more tech powers.

The only abilities that can detonate multiple enemies are dual throw, charge with radius, shockwave, trip mine with detonation upgrade, and the drone missile upgrade. Shockwave does reduced combo damage, and I'm not sure if throw still does reduced combo damage (it was changed to do full damage in multiplayer but I don't know about single player).

The engineer profile is not particularly beneficial unless you're using constructs. The sentinel is good for combos and survivability. The adept is specialized in biotic aoe combos and so will do more damage with biotic combos against groups but I believe equal or less than the sentinel against lone enemies.

Singularity
Lance (shield powered)
Energy drain (or other tech power)
Omni vents
Shield oscillator in chest

Singularity and energy drain are both universal primers, omni vents makes energy drain greatly reduce the long cooldown on singularity, lance is the most spammable detonator, energy drain is a backup detonator that you can use if your shields are down, and both the shield oscillator and energy drain restore shields. This build benefits from everything in the sentinel kit but also works well with the adept and explorer.

I recommend using the fusion mod of adrenaline eventually. It pairs extremely well with exploding singularity, but it also makes pretty much all primers and detonators spammable as long as you get kills.

is it possibe to chain oobo with cryo beam - incinerate - energy drain?
incinerate can prme and detonate but i don't know if it can do both things using it once