Dies to Removal

Lots of good advice here. In particular, I think you're dead on that Riku's most important spell slots tend to be in the 4cc slot, such that they can be doubled immediately after Riku is played.

In building my own Riku list, I initially found that the list was underperforming compared to my other decks, probably somewhere around the bottom 25th percentile (I'm up to 20 EDH decks currently assembled at the moment, with some help from proxies to duplicate cards that I use in multiple decks).

The reasons why seemed to be that:

1. Whenever Riku comes down, it has a very, very large target on its head.
2. Red and green don't have great options for board sweeping and removal. Although there are a number of good options, such as the usual suspects in blue and various odds and ends in red/green, you can get overrun quickly compared to decks that also have access to white and/or black for support.
3. Riku tends to be
slow. It takes a good long while to charge up to enough lands that you can safely play and protect Riku, but in the meantime, your lack of defense means that Riku tends to be on the ropes before your mana base has reached an ideal level of development.

To tackle those problems, I tried a couple things.

First, I crammed in as many ways as I could find to easily cast Riku on third turn, such as Devoted Druid, Lotus Cobra, Exploration, Burgeoning - single cards that put you two slots of acceleration forward by turns 1-2.

Second, I changed around the deck concept to emphasize blowing up lands, enchantments, and artifacts. Stuff like Creeping Mold, Rain of Salt, Reap and Sow, the Crucible/Strip Mine/Wasteland combo. It was silly, but EDH decks tend to be very mana hungry, and a lot of those cards could be played to break down other players' acceleration draws (Sol Ring, Worn Powerstone, etc.), cripple mana bases, eat up their counter-magic, and generally slow down their progress to the point that
your own acceleration plus Riku's natural card advantage engine would have time to get set up with a nice lead. Putting enemy players off the colored mana they need to deal with Riku is pretty huge. Many of your land destruction spells are in the 4cc slot and easily doubled, so nuking two lands per turn once Riku hits the table is pretty standard.

It actually worked out pretty nicely. Multi-player is more of a challenge, but if you're blowing up between two and six lands/permanents per turn with things like Rain of Salt, Natural Order into Terastodon hijinks, Godo/Argentum Armor, you can feasibly cripple several players at once, then finish off whoever is left.