The particle/postposition に in the construction above is used to stating the purpose of an action.
"[Someone=The speaker] wa [someone else=なっつん] o [shiage] niwachōhō shite iru."
'[The speaker] always uses [Nattsun] as a convenient tool for doing the finishing process.'
* The speaker is talking about Nattsun in a disparaging way. (see 口は悪いけど[...] above)