The easy way to do this is use Limbo build for Duration + Range (see example build below). Akkad on Eris (Infested Defence lv 35-45) is a good choice for this Riven Challenge.

I recommend doing the mission solo. Before you approach the mission target you should fill up your energy bar with an Energy Restore so you are prepared to use you 2+4 combo immediately. You may also drop some Specters to assist you if you want to make this challenge even easier.

When the first wave starts immediately cast Limbo's 2+4 combo: the Stasis and Cataclysm abilities. With the build shown below the Cataclysm should be up for the entire wave; however you must keep an eye on the timer on the Stasis ability and recast it when it times out - you can also cancel it and recast when the timer is nearly out and there are no enemies nearby to avoid the risk of enemies damaging the mission target while you recast Stasis.

Assist your Specters in killing any enemy that gets frozen in your Cataclysm bubble.

When the wave ends cancel you Stasis + Cataclysm by pressing the ability buttons again - then wait for the next wave to start and cast your 2+4 combo again. Repeat until you can exit mission after wave 5.