This study presents a new class of Er/Yb specialty optical fibers designed with a focus on high-power laser and optical amplifier applications. Utilizing an octagonal cladding design and a composition of multielement-doped silica glass (Al, Ge, P, Ce, B, F), we achieved remarkable lasing efficiency of over 41.0%, alongside stable output power and commendable radiation resistance. The optimization involved adjusting key parameters, including glass composition and the deposition temperature of the porous fluorinated phospho-silica core layer, through a modified chemical vapor deposition (MCVD) method combined with a solution doping technique to ensure uniform integration of the dopants, effectively mitigating clustering issues. The optimized doping ratios (7.5–11.1) of Yb2O3, and Er2O3 along with fluorine, GeO2, CeO2, Al2O3, B2O3, and P2O5 significantly enhanced fiber performance. We experimentally explored different cladding geometries (double-D, pentagonal, hexagonal, and octagonal) and evaluated their lasing capabilities. An octagonal cladding-pumped fiber demonstrated an output power of 21.0 W at 1558 nm, achieved with 52 W of absorbed pump power at 940 nm. This specific fiber structure contained 22.25 wt.% P2O5, 2.43 wt.% GeO2, 0.5 wt.% Al2O3, 0.161 wt.% CeO2, 2.41 wt.% B2O3, 0.25 wt.% F, 0.515 wt.% Er2O3, and 5.25 wt.% Yb2O3. Performance characterization revealed a gain of approximately 28.0 dB with little variation (less than 0.45 dB flatness) over a wavelength range of 1540 to 1570 nm for a 10 dBm input signal. Additionally, under prolonged operation exceeding 150 h, the fiber maintained an average output power of around 15.0 W, with parasitic lasing below 1.0 μm and less than 10% degradation in output after exposure to 6.0 Krad of cumulative 60Co gamma radiation. These results underscore the potential of this multielement-doped silica glass fiber in applications requiring robust performance and stability in challenging environments, such as free-space communication systems.