That mission can definitely be a tough one. You need to make as fast a start as you can, as the first strong teal hero is scripted to appear at the start of month 2, week 2. After that you have a similar strong scripted hero showing up every week. For the first couple you may consider hiding out in your castle and getting them to attack you there, as that can provide the edge needed to beat them off the first couple of times. You'll also need to keep casualties low during these fights- since the enemy heroes are scripted battles of attrition will quickly leave you much weaker than the heroes attacking you. While defending against these attacks build up your forces as much as possible, because the next phase of the mission is to take to the seas, and there are a couple of strong dark elf armies you need to fight there. When you take to the seas you'll basically need to abandon your existing towns, as you won't be able to defend them well with weak backup heroes. Your main hero will need to proceed across the ocean to the northeast, where you'll find another town to capture (teal won't follow you here, so once you get there you can safely build up your forces). From that point the tough part of the scenario is over, and you just need to build up your forces until you can take on the final enemy hero you need to beat.
