Above you're seeing the standard late-game build out of a Warrior/Hunter opening that focuses on the full (6) Warrior synergy. It was one of our best-performing builds in our late-game strength test (as seen in our Best Underlords Builds Tier List).

In this variation, two-stars on Medusa and Troll are a big power-spike since they are your main damage-dealers, which means that pushing for lvl9 or 10 earlier rather than later is a good idea. If you get to lvl10 you usually want to get a second Troll for the (2) Trolls attack speed + the mini-stun bonus. Going for (4) Scaled is also possible if you're facing Mages.

At the same time, three-stars on Tiny, Slardar, Wind, etc., are also great. This means that if you don't have the luxury to push for lvl10 you can certainly do well by rolling for these upgrades earlier. That said, lvl9 is still crucial because on lvl8 the chance of finding Medusa + Troll is significantly lower.