Diamond discs

You should at least wear a professional respirator, not only in dry cutting, but also in wet cutting, because the small dust particles generated in metal cut off wheels process can penetrate deep into the lungs and can cause serious health effects such as lung cancer or silicosis. If the job is big, other equipment like goggle, earplugs, and protective clothing should also be adopted.
To make the cut straight, you can use a chalk line to mark a straight line on diamond blade, and then place a straight wood board along the line. Then you can cut a shallow line on the concrete along the wood board. The cut can be about 1/4″ deep.Having this shallow straight metal cutting discs, you can perform further cuttings along it to gain the cut you need.