I am greater than you!
Python 2, 85 80 77 72 70 bytes



lambda l:''.join(sorted(map(str,l),key=lambda i:i+i[-1]*max(l))[::-1])


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Sorts the numbers lexicographically, but each number is padded with its last  digit.

This means that 'shorter' numbers (string-wise) can be larger than 'longer' numbers:

Example:

Input: [76, 7]

Each number gets padded with its last digit: ['76666..','7777..']

Sorted (descending): ['7777..',76666..'], which gives [7, 76]

Joining the result gives: 776