Focusing of multimodal beams by chains of dielectric microspheres assembled directly inside the cores of hollow waveguides is studied by using numerical ray tracing. The device designs are optimized for laser surgery in contact mode with strongly absorbing tissue. By analyzing a broad range of parameters it is demonstrated that chains formed by three or five spheres with a refractive index of 1.65-1.75 provide a two-fold improvement in spatial resolution over single spheres at the cost of 0.2-0.4 attenuation in peak intensity of the central focused beam. Potential applications include ultra precise laser ablation or coagulation in the eye and brain, cellular surgery, and the coupling of light into photonic nanostructures.