Here's what rhymes with frustratingly.

Consider these alternatives

excruciatingly / unhesitatingly agonizingly / antagonizingly exasperatingly / unhesitatingly annoyingly / cloyingly tantalizingly / surprisingly curiously / furiously elusive / inclusive maddening / happening imprecise / price laborious / glorious irritatingly / unhesitatingly sketchy / already disjointed / disappointed mystifying / dying pathetically / theoretically