My unit composition was only mounted archers for the two first bases and only archers for my base.
I started by taking the two additional gold mines, and making a lot of wisps into Jaina base. Then started building a lot of ancients of war inside the base and covering them with moon wells.
Then, starting upgrading the archers and building two ancients of war and wind behind the human base, to continuously send reinforcements, and using additional wisps to auto repair the structures.
I don't know the exact timings, but when I started to get overwhelmed, I flew to the orc base and used the same strategy, but I had less space for ancients of war, but I had time and gold to make more mounted archers (and they where already fully upgraded at that point).
Around 13 minutes left, I backed up to my base and due to the lack of gold, I only made ancients of war (not moon wells), and normal archers. When I was capped or nearly capped, I built some Tree of life's blocking the access to the door (that always work), and using all the buildings that were not producing to block the path.
I had quite the trauma with this mission and the last one of the Undead. This one was REALLY hard to me as a child on normal, and last time I played the game on Hard, I got struck with the Undead mission. I didn't thought that I would win the first time, and less with this strategy, but it worked. I had plenty of time at the end, I ran out of gold but my units where still up, and the buildings blocking the path where intact.