You could hire a stihl saw which has a 12" blade and would cut easily through the rest ,or hire a kango and break the concrete up to your line and with a bolster blade inserted chip away at the concrete to form your straight edge ,wont be perfect edge but if it needs to be could mix up some mortar to make good.
Hope this helps
You could hire a 12" petrol saw (Stihl saw) that will cut deeper and better, use a diamond blade and correct PPE.
When cut to max depth you can then break the last bit with a breaker or a sledge hammer.
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