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My boss unilaterally decided my work was sloppy.
Literal
Boss [topic-は] my [genitive-の] work-way [subject-が] sloppy [quotative-と] decided-attached.
いい加減 is a versatile word with two opposite-feeling meanings: 'moderate, just right' (when positive) and 'sloppy, half-hearted, careless' (when negative). Here it's clearly negative. 決め付ける ('to decide unilaterally, label, jump to a conclusion') is a compound verb: 決める ('decide') + 付ける ('attach') — 'to fix a decision onto.' Expresses a criticism of a judgmental boss.