Matata has written a brilliant little guide over on Reddit, where he discusses 5 key points that helped him significantly improve his Dota Underlords play. We've transcribed his thoughts below (he's not a native English speaker). 

#1 Stop Trying To Win Without A Level 2 Unit or Alliance Synergy

At the start of the game, you should either be win streak or lose streaking, and nothing in between. Their is nothing stoping you from analysing your oponnents boards, your board, and figuring out if you can beat them or not. As a rule of thumb, if you do not get a LVL 2 by round 4-5 pursue a lose streak. Health is a resource so use it to leverage your economy to the mid-game; it's best to take a 40hp loss in the first 10 rounds, than to find yourself lose streaking in round 25, where the damage is significantly higher. Finally, don't spend gold leveling up before the-mid game, unless you are going for a win streak.

#2 Stop Re-rolling Before 50 Gold

Don't worry about board control in the early game! Your opponents that are going for re-rolls are going to have stronger boards. All you then have to do then is to pursue a lose streak. The gold they use for rerolling will quickly vanish and yours is safely kept, generating interest. gold in this game is the ultimate ruler: it will decide whether you're going to lose or win your games. Focus on your gold management. If you need to sell a unit to hit an interest goal, do it.

#3 Stop Buying Before the Batlle Starts.

The game registers your gold total at the start of every fight. So if you wait until the battle starts before buying those two 5 gold Gyrocopters, you will save 1 gold of potentially wasted interest. You can use this when you are either at any gold value, and you can also use it to level up so that the next "pack" you open is higher Level.

#4 Stop Going Into The Game Without A Plan

Before the game starts, you should have in your mind multiple builds that you might want. Right now, Knights and Trolls are king, while Warrior/Hunter compositions also do well. Spy on your ennemies to see what they are picking and see what your first 3 items are. By Round 15 you really need to have comitted to a composition.

#5 Stop Picking The Same Heroes That Your Opponent Has 2*

Did you know that the units you are picking in the shop are not infinitly generated? There is only a fixed number of units, and you share them with other players. So if you see that someone has a level 3 Batrider just forget about it and go for something else instead.