As for U.Sub-Zero tips, ice clone is your best friend. Utilize it defensively. And front ice shower (d, f, b, HP) is very useful, especially when you know your opponent is about to jump forward out of the way of a regular ice shower (d, f, HP). It usually works most of the time, just read your opponent well. And a great combo (that I like to use at least) when they are frozen in the air is: HP, ice ball (the timing is tricky, make sure the first HP connects), then HP, HP, forward JK, slide. I don't remember the exact damage output, but its fairly decent.
Defense is U.Sub's stronghold, in my opinion. Hold and read your opponents down and punish as much as possible and you'll be good to go. Hope these tips helped you out some. Hails.