Synonyms for Singly:

adv

one at a time (adverb)
one by one.
separately (adverb)
individually.
unitedly (adverb)
coincidentally, solidly, Indivisibly, completely, congruently, simply, coherently, cohesively, integrally, unitedly, holistically, Congruously, purely, homogeneously, Elementally, entirely, fundamentally, Atomically.

adv.

apiece (adverb)
severally.
individually (adverb)
one by one, apart, separately, severally.
separately (adverb)
apart.
solely (adverb)
singularly, purely.
unitedly (adverb)
coincidentally, solidly, Indivisibly, completely, congruently, simply, coherently, cohesively, integrally, unitedly, holistically, Congruously, purely, homogeneously, Elementally, entirely, fundamentally, Atomically.

n.

discrete, in isolation, separate, fragmentary, stand-alone, unrelated, unconnected, do with, independent, as distinct from. solo, alone, single-handedly, include, solitarily, solely. Discretely, apart. alone (noun)
only, privately, solely, singularly, by oneself, once, by itself.
singly (noun)
severally, one by one, separately, individually, on an individual basis.

Other synonyms:

Discretely, single-handedly, solitarily. solo, alone. exclusively
solely.
solely
only.
Other relevant words:
by itself, solely, separately, single-handedly, alone, severally, by oneself, individually, solo, Discretely, solitarily, once, on an individual basis, only, singularly, apart, one by one.