Kerbal Space Program: Getting the SRB-KD25k "Kickback" to orbit?

The trick to make this easier is to lower the amount of solid fuel in the booster when you build your ship.

The S1 SRB-KD25k "Kickback" Solid Fuel Booster has a dry mass of 4.5 tonnes and is capable of carrying 2500 units of solid fuel.

Solid fuel has a density of 0.0075 tonnes per unit, therefore a fully loaded booster carries an additional 18.75 tonnes (2500 * 0.0075) for a total mass of 23.25 tonnes.

In the VAB, right-clicking the booster opens a menu that allows you to select the amount of fuel it contains. This value can be lowered from 2500 to 250.

Doing so brings the total mass of the booster to 6.375 tonnes (4.5 + 250 * 0.0075). That's a little under 30% of its original mass.