November and December are the best season of the year for Bouldering in Hampi. Avoid the rainy season and the peak summer.
Hampi is hot and dry. I mean the summer temperature can go past the mark - 40 deg C ( 104 degF). Summer pickups momentum from February onward. March to June is really hot. Then the monsoon (rainy season) arrives, making the boulders wet and slippery. By August monsoon recedes but the heat still persists, though on the decline.
November and December are the coolest months for Hampi with pleasant weather.
You may stretch your plan on either side of this 'season' a bit, if you don't mind a bit of the hot climate.
One way of managing this is to start early (say by sunrise) to the boulder fields. Retire when the heat reaches beyond your tolerance. Take a long mid-day break. Start again in the evening, till sunset.
Also select the boulders with problems on the shade side, depends of the time of the day. I mean attempt problems that are located on the western sides of the boulders during the forenoon and vice versa in the afternoon. This hide and seek strategy can somewhat minimize the direct exposure to the sunrays. Nothing can prevent an inspired boulderer from attempting whatever he/she likes. Take care of yourself anyway.
Climate chart for Hampi