Surely they'll have rudimentary mining tools to aid them. Nonetheless, if killing or capturing a giant proves too difficult than we could free and rescue a few dwarves during some night raid and question them as to the artifact's status. Than, depending on the information, we could try and raid the camp again or devise another plan, or deliver the bad news that giants already took it off somewhere to Baldur. Additionally, rescuing a dwarf and questioning it could gain us insight on the strengths and weaknesses of what the inside of their camp looks like.