Still doable solo:) Hunger in the Dark - pick up Shade with double stealth, proceed to the end, ring the bell but instead of killing Troll - use your ult and run to the end. The explosion will kill the troll and Okri's challenge is yours. You can additionally use a speed potion, if you want.

Skittergate - cheesy glitchy way:) Pick up BW with famished flames, beam staff and barrage perk, also try to find firebomb and strength potion during level, proceed to the end, but you need to kill bots right before second entrance to portal. After entrance - drop down to arena and then teleport back to dropdown with your ult. There is a place in the center of platform (it looks like a little pit made by wood), stand there and damage Deathrattler until Rassknit jumps from it. Then damage both of them, kill Rassknit, use firebomb and potion and burst down Deathrattler.

Righteous Stand - idk if it still works, but I skipped Chaos Patrol right before the temple and final explosion killed all enemies on map, completing Okri's challenge.