Antonyms for the word "lightly-armed" would include heavily-armed, heavily-armed and armored, heavily-fortified, and heavily-defended. Other antonyms might include well-equipped, well-stocked, well-supplied, or well-prepared. These opposite terms suggest a sense of security, strength, and preparedness, rather than the vulnerability and weakness implied by being lightly-armed. A well-prepared army, for example, might combine a range of weapon systems and tactics to provide maximum defensive capability, while a lightly-armed force might rely on speed, mobility, and agility to evade enemy fire and launch quick, unpredictable attacks. Ultimately, the choice of approach will depend on the specific situation and circumstances of each conflict.