"assertiveness of" is a grammatically correct phrase and can be used in written English.
For example, "John demonstrated an impressive level of assertiveness of his opinions at the meeting."
Even the proudest and most assertive Russian and Soviet statesmen have shown, sometimes by the very assertiveness of their pride, that they long to be taken seriously by more established members of the club.
An assertiveness of touch tends to go along with that.
The experience of light was enhanced, but so was the physical assertiveness of the painted surface.
The result has been a new assertiveness of national identity, and confidence in their culture.
It's a gesture of defiance, signalling the new assertiveness of LGBT Ugandans.
However baseless Cameron's economic assumptions, nothing seemed to dent the assertiveness of the deficit hawks.
"Part of it is just the greater assertiveness of Turkey's foreign policy everywhere".
Iran could also benefit from the growing assertiveness of Shiites in general.
It's not that they fail to acknowledge the assertiveness of the teen female.
The increasing sexual assertiveness of Spanish women has also contributed to the Viagra trend.
But on the number and assertiveness of the inspections, they say they have nothing to apologize for.
