Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
agmĭnātim, adv. [agmen], by troops, in trains, in crowds, = gregatim (only in late Lat.): elephanti oberrant agminatim, Sol. 25; App. M. 4, p. 151, 35 Elm.
Trop., App. M. p. 146, 2 Elm.