Copied over from MTGSalvation. (Still my design, though.) This deck has two basic ways to win: build up one or more of the unblockables (River Sneak) to punish the opponent, or build a swarm of hexproof tokens oriented toward smashing through the defenses on a turn III Song of Freyalise, or some combination of both.
Avoiding mana drought: mulligan any hand with only one land unless you just can't pass up the other cards and you feel lucky. With two lands, look to get one or more low casting cost creatures onto the board and follow them up with a Song of Freyalise. However, if you're not having mana issues and have a Song in hand, hold it up. It'll be better used later in the game.
Get out a Deeproot Waters as soon as possible. Then work the tokens, drop a Song of Freyalise, and go to town on turn III. +1/+1 counters should generally go to the unblockables unless there's another overriding reason, like building a solid blocker or pumping toughness 1 creatures to avoid Chainwhirler or Blazing Volley kills and such. (Mistbinder is the preferred way to deal with that threat, though.)
Use the Tricksters for disruption of defenses; use Blink of an Eye to return one of them to hand or to disrupt opponent's blockers. Another, more expensive option is Jungleborn Pioneer, with commensurately greater reward. Remember, each time you can get to reboard a Trickster or Pioneer is an additional token from Deeproot Waters and/or a +1/+1 counter from Deeproot Elite, two in the case of the Pioneer. And with both Deeproot Waters and an Elite out, a single Pioneer is worth three creatures (itself, its token, and a token from Deeproot Waters), and a counter per from the Elite. The potential for abuse is only limited by your mana. Just don't get overzealous and tap yourself out. Blink is a great way to bleed disruption from your opponent, too.
Sweepers are your mortal (pun intended) enemy...Hour of Devastation, Settle the Wreckage, Goblin Chainwhirler, and their ilk. Either make sure you save a counterspell or Blink 'in case' or be very judicious about what you attack with...preferably both. (As mentioned above, a Mistbinder will do against a Chainwhirler.)
*Updated for rotation. Unfortunately, I can't and won't recommend either this deck or my other UG merfolk deck (Fileted by Fish) until at least Ravnica Allegiance. GRN took away and gave nothing back. I don't think it's competitive enough anymore.
