"He speaks with the guard."
Translation:Lui parla con la guardia.
As this Treccani entry points out, "Oggi le uniche forme ancora utilizzabili (anche se non molto comuni) sono col e coi" (nowadays the only still usable forms, albeit not very common, are col and coi); however this other Treccani article states that all those forms are admissible, and natural in speech (which I agree with), and yet another Treccani article simply states that "colla" (cólla) is more frequent in speech than in writing.
Confusion aside, "colla" is not wrong, but usually avoided in writing, and most grammars discourage its use; although the reasoning that it's because it's homograph with "colla" (còlla = glue) is pretty feeble as there are many more that aren't avoided at all.
Bottom line, don't merge con with articles in writing, but you're free to do it when talking :)