(Totally not sorry at all that I was thinking of this when you said coffin.)
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[img]http://i.imgur.com/LzitSIg.gif[/img]
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(Or maybe something like this?)
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[img]http://vignette1.wikia.nocookie.net/owarinoseraph/images/8/81/Kiseki-o's_Box.jpg/revision/latest/scale-to-width/2000?cb=20150108014246[/img]
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Anyway, I was thinking that since it's hollowed-out inside, it can act as a storage of some sort - that is, anything pulled out from inside can be the weapon, not the coffin itself. So if you want it to shoot dirt, stone pillars or even vines, then maybe those have to be kept inside the coffin first before they can be utilized? Of course, the shielding part makes sense tho'.

Originally I thought you might go with spear or boots since in certain fantasy works they can be attributed to the earth element (otherwise spear can go with water using Poseidon's trigon as reference, and boots to air if you think of Lenalee in DGM), but coffin is sure an interesting idea.