We numerically study the impact of a large sphere dropping into a prefluidized granular bed using a state-of-the-art hybrid discrete particle and immersed boundary (DP-IB) method. For the first time, both the gas-induced drag force and the contact force exerted on the intruder are investigated separately. Our results show that even for relatively large granular particles of 0.5 mm diameter, namely Geldart B particles, and an intruder of 1 cm, the drag exerted by the interstitial gas accounts for up to 5% of the total force experienced by the intruder. Our simulation results match well with existing experimental observations. This work shows that the current simulation scheme could become a tool to investigate the effect of interstitial gas on the dynamics of projectile impact cratering. More generally, the method allows for accurate simulation of the hydrodynamic effects of large internal objects moving through (pre-)fluidized granular beds.