。
She called out to us loudly for help.
Literal
She [topic-は] us [to-に] loud-voice [in-で] help [object-を] sought.
助けを求める is the standard 'ask for help, call for help' collocation: 助け ('help, rescue') as object + the verb 求める ('seek, request, demand'). 大声で uses で as the means/manner marker — 'with a loud voice, loudly' — alongside the addressee marked with に ('to us'). The whole sentence is the kind of thing you'd see in a news report or narrative passage describing someone in a moment of distress.