An entangled-state generation protocol for a system of two qubits driven with an ac signal and coupled through a resonator is introduced. We explain the mechanism of entanglement generation in terms of an interplay between unitary Landau-Zener-St\"uckelberg (LZS) transitions, induced for appropriate frequencies and strong amplitudes of the applied ac signal and dissipative processes dominated by photon loss. In this way, we find that the steady state of the system can be tuned to be arbitrarily close to a Bell state, which is stable against photon loss. Effective two-qubit Hamiltonians that reproduce the resonance patterns associated with LZS transitions are derived.