Autofill Dynamic Range

I'm creating a macro that fills a range that changes every time it is run. The only thing consistent about the range is that it begins on row 7 of the active cell. The column filled changes every time. The number of rows may change. Therefore code like the following will not work.

Range("A2").AutoFill Destination:=Range("A2:A" & LastRow)

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I need something more dynamic like this.

ActiveCell.AutoFill Destination:=Range(ActiveCell.Address & ":" & ActivceCell.Column & LastRow)

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That will not work because the column property of the active cell returns a number, not a column letter. I maybe could try to convert the number to a letter, but it gets complicated after the letter Z.
Is there any way to auto-fill a completely dynamic range? (Well, as close to dynamic that I can think of.)
I also have tried substituting the Cells function inside of the Range function. No go.