The key to completing this chapter with ease is to push aggressively and fast. I had to restart this mission twice because the first two times I tried going at an easy pace and destroying all the mini bases before heading to the main green base and this gives the main base a lot of time to build more units and become stronger. As time goes by, the aerial attack by the green base also increases in strength which means you need to increase defense at the base, reducing your army in the offense. Even though there are 4 gold mines, you will soon be out of resources if you advance slow in this chapter.
Strategy that worked:
1. In the initial phase, when all the elf bases in the north and south are being attacked, move all the villagers and army (there will be 1-2 people in each base) to a corner of the map, where the undead will not find them. This destruction of bases is scripted so there is no point in fighting the scourge at this point in time. Once the cutscene ends, all the saved villagers and army will the teleported to your central base, giving you more army, saving both resources and time in the early stages.
2. Leave Vashj at the base with the elf archers for defense and take Kael along with the naga mirmidon’s and some archers to the purple base in the south east corner of the map. If you decide to keep some of the naga mirmidon’s at the base for defense don’t forget to use their ensnare ability to web the flying enemies. Take one dragonhawke rider to prevent the towers from attacking your troops. This base should take very little time to be destroyed. When you break the crates in the corner, you will find a tiny castle which you should save for later.
3. Get a few villagers here to build a town hall and start mining gold. Nothing else is needed here at this point.
4. Take Kael’s army after adding reinforcements, if necessary, and move to the purple base to the west of the newly established base of operations. This base would have slightly more troops, especially spiders, so take more than one dragonhawke rider. Once this base is down, create a town hall using the ‘tiny castle’ that you found when you destroyed the south east base in step 2. Ask a worker to build towers and move Vashj’s defensive army here.
5. Build more troops and create a bigger army and attack the Green base. Make sure not to cross into high upkeep. I was still at 51/90 upkeep when I attacked the green base and did not have any issues defeating it. If you haven’t spent more than 45 minutes before attacking the green base, this should be easy to defeat.
There is no need to destroy all mini bases (orange and purple) before attacking the green base. Note that when you’ve destroyed the purple base on the west, the orange base in the north west will send some troops to attack you in the south. The strength of the army will depend on how much time has passed since the chapter began.
I’m sure there are other strategies that work, however, the key is to attack aggressively and fast before the enemy gets too much time to strengthen his army.