"focusing mostly on" is correct and can be used in written English.
For example: "When I studied the material for the exam, I was focusing mostly on the concepts I felt I needed to review more thoroughly."
Even in traditional proofs, reviewers rarely check every step, instead focusing mostly on the major points.
It does not endorse democracy or take part in elections, focusing mostly on preaching.
Not for a wage, but as an extension of her old community outreach work, focusing mostly on women's issues.
CNBC has been focusing mostly on business-oriented talk programs at night, like "Kudlow & Cramer" and "Capital Report".
Ms. Hahn, who joined the company shortly after winning the championship, is a senior trader, focusing mostly on commodities.
Focusing mostly on corruption and the justice system, critics rarely addressed Romania's core problem: the profoundly dysfunctional political process.
They're focusing mostly on prevention, which is smart because prevention can save thousands and thousands of lives a lot faster than curative medicine".
These place large sections of its guidebooks to eight cities, focusing mostly on food and drink, on to iPods, complete with full-colour images and maps.
Initially, the company will start out small, with six dancers performing two weeks each month, but focusing mostly on teaching classes and summer workshops.
iPad Developer Tinybop made the excellent The Human Body children's app, but this is the follow-up, focusing mostly on flora, with a spot of fauna too.
In another unusual move, Mr. Romney has also been running advertisements on national cable networks, focusing mostly on Fox News, a favorite among conservatives.
