http://archive.wizards.com/Magic/magazine/Article.aspx?x=mtg/daily/stf/45

Protean Hydra + non-counter-based toughness = damage immunity. This is where it gets a little abstract—and very powerful. When Protean Hydra takes damage, its middle ability does two things: it prevents that damage, then it removes that many +1/+1 counters. If the Hydra takes more damage than it has +1/+1 counters, it prevents all that damage, and then removes as many +1/+1 counters as it can—namely, all of them. However, if the Hydra still has toughness from a source other than +1/+1 counters—i.e. if its base toughness is greater than 0/0—then your Hydra won't die.

Aura here might be Uncontrollable Anger—either you can force your opponent's creature to attack into your Hydra, or you can give your Hydra a surprise +2/+2, giving it damage immunity at a crucial time.

another route to go would be with red so you could cast things like earthquake destructive force and pyroclasm to pump him huge while blitzing the opponents board