Why C6 and C3 that are (more or less) on the gate of each MOSFET. Remove them.
Pin 3 & C5 should not go to 15V2. There needs to be a diode from 15V2 to Pin 3 and C5. (high voltage, fast, low current, see data sheet) This why you are killing the gate of the top MOSFET. (the IR2113 might be dead also)
The supply for LO is from 15V2 and that is OK.
The supply for HO sits on the output of your two MOSFETs. It swings from 0 to +300 volts. When it is at 0 volts current from 12V2 passes through a diode and charges C4. When the output is at 300 volts the diode cuts loose C4 and allows it to 300 volts. The HO driver lives on the stored current C4. It is not powered by 15V2 at this time. With no diode VB, HO, VS can not to high and something brakes.
Hope this helps.