Bonus do not stack but in this case both are not bonus.
Do thei even work together? (can you reduce the energy resistance by 15)
penetrating Attack (core):Reduce your target’s energy resistances and damage reduction by 5 against your weapon attacks.
Unstoppable strike (armory): If your attack with
a weapon does not already overcome the target’s energy
resistance, treat the target’s energy resistances as though
they were each 5 lower. At 15th level, treat the target’s energy
resistances as 10 lower.
You reduce the resistance by 5 then, it did not overcame the enemy resistance so i treat it as being 10 lower? Does it work that way?
I've found no reference in the core book how to work with this.
Heavy Onslaught, does the samething as Unstoppable strike, but against damage reduction, it will probably work the same way.
They have different timing. Penetrating Attack reduces Resistance and DR by 5. If that removed all of the Resistance, then there is no further effect. If they still have Resistance after that, then you reduce by 5/10 more.