You need to decrease the temperature of the corona
A solar flare is caused by magnetic reconnection (probably; I will assume this is true for the rest of this answer). A series of magnetic loops near each other will reconnect with each other in a different topology. Since highly energetic charged particles tend to follow the loops, this reconnection can orphan portions of the magnetic lines of force. These unconnected magnetic helical fields and the charged particles tracing them can then explode outwards in a flare.
Magnetic reconnetion occurs when two (or more) magnetic field lines approach each other. The plasma's electrical resistivity in the boundary region opposes the currents that should be formed by the two fields. This opposition forms a 'potential gap' of sorts, acting in a way like a magnetic capacitor (please don't quote me on that to a physics professor). This is the mechanism that stores up energy. Once the magnetic lines do reconnect, this energy can be dissipated into the 'orphaned' charged particles, giving them their motive force.
Since magnetic reconnection is associated with higher electrical resistivity, in order to induce flares, we want to raise the electrical resistivity. Resistivity is the inverse of conductivity. Figure 2 here shows that electrical conductivity increases with increasing temperature (up to 30,000 K at least...). Therefore, electrical resistivity decreases with _increasing temperature.
Therefore, anyone saying to hit the sun with a lot of energy to cause a solar flare is probably wrong. You want to decrease the energy in the sun's corona along the magnetic field lines. This will lead to more stored energy that can be released by magnetic reconnection.
How to decrease the temperature of the corona? Frankly I have no idea, not even in far out sci-fi terms.