I play him, my suggestion is to maximize mana ramp and card draw. Riku has an interesting conflict where to maximize his power you need a lot of mana, but if you have that mana its hard to have cards in your hand.
Sphinx of Uthuu, Consecrated Sphinx, Primeval Titan, Sensei's Top, Lotus Cobra, Yavimaya Elder: this is a good support suite. In my deck I run azusa, lost but seeking, oracle of muldaya, and exploration (this is irl) as well. Your goal is to cast riku on turn 4 and have 9 mana on turn 6.
Getting there: Riku has a lot of trouble getting there sometimes, here are some affective ways:
1. Tooth and Nail:by far the most effective way, a copied entwined tooth and nail will win you the game most of the time. Fetch Terastodon, Kiki-jiki, Blightsteel, and Deceiver Exarch. Some play groups ban infinite combos or just outright ban Kiki-jiki, in that case fetch Teferi, Seedborn Muse, Consecrated Sphinx, and Primeval Titan/Terastodon.
2. Copied Murkfiend Liege +Copied Overrun. With board presence of Riku, 2 Liegies and a random U/G card like Coiling Oracle or Intet or mystic snake you can deal 40+ damage this way in one turn.
3. Copied Primeval Titan-> copied comet storm level mana. If you can up to around 25 mana you can use comet storm to burn the field out, as riku will copy the kicker.
Also Riku is a creature based deck, the spell based version is a lot less good.