The correct spelling of the phrase "study at heavy" can be explained using the International Phonetic Alphabet (IPA). The first word, "study," is pronounced as /ˈstʌdi/ with stress on the first syllable. The second word, "at," is pronounced as /æt/. Finally, the adjective "heavy" is pronounced as /ˈhɛvi/ with stress on the first syllable. Therefore, the correct spelling of the phrase is "study at heavy," and its pronunciation can be accurately described using the IPA phonetic transcription.