Is there a guide or written walkthrough anywhere? I feel like I'd be having an easier time with some of these missions if I had some good strategies. I'm stuck on Amplification right now.
I just got through that one, so I can help.
Right away build a Sanctuary of the Sun and an Arcane Vault. Mass produce Spell Breakers whenever you have the money for them. The Wretched from the south and north only produce Sorceresses, Swordsmen, and Arcane Goldems, while Dar'Khan comes with Skellies (I believe he summons them with an ability, so Feedback does bonus damage), Banshees, Arcane Golems, and Crypt Fiends from the west. The casters and Golems can't do
anything to Spell Breakers, and Feedback is instrumental in quickly dispatching the otherwise pretty tanky Dar'Khan and Arcane Golems. The first few minutes is pretty slow but by around 15 minutes left things get pretty hectic. Do not try to replace lost swordsmen, because the Arcane Golems just shred them.
Once your situation is a little stable, build an Arcane Sanctum, upgrade your priests and sorceresses, and produce a few sorceresses. Use their silence skill on groups of enemy casters when you see them. There's another gold mind to the west of your base guarded by a high-level golem, take him out and claim it if you have the time. When you have the time and resources for it, mass Arcane Towers at your three entrances as well. They're effective against enemies the same way Spell Breakers are. Lor'Themar should hit level 6 after a few waves, save it for Dar'Khan when he shows up, but he doesn't seem as aggressive as the Wretched in this phase. You should try to build up to 2 control groups, which should be pretty easy if you focus on Spell Breakers. Also, get your melee weapons and heavy armor upgrades out. Rangers also aren't bad since they help Breakers dispatch the mages pretty effectively and they tend to stay pretty safe thanks to their range and Breakers forming a good front line in this mission.
Once the defense portion is over it gets quite a bit easier since you get Kael and Alessandra, all their sorceresses and priests, and 3 arcane golems for free. Your starting gold mine is timed to run out right about now, so go claim the one guarded by the golems. Once your base has enough arcane towers to weather an attack, take your forces through the south exit and wipe out the Wretched base in that direction, it's pretty poorly defended. Just be careful not to let your Arcane Golems eat Arcane Tower shots. The north Wretched base is harder to crack since the only entrance is very narrow, but just throwing a lot of units should do the trick honestly: Wretched don't have much in the way of a tech tree. With those two bases clear the pressure will be much lower. There's an easy purple Scourge base (it doesn't ever send attacks) with a gold mine in it on the way to the north Wretch base, and there's a secret banshee hidden behind some trees in the south Wretched base that drops a Crown of Kings if you select her.
For Dar'Khan's main base you need to cross a cramped swampy area with purple Scourge defending, I suggest taking a smaller group of mostly Spell Breakers and your heroes. Be careful not to get caught by an attack from Dar'Khan while you're doing this. Dar'Khan's main green base isn't heavily defended, but it's very cramped and half of it is on high ground. Build up 4~5 Arcane Golems and have them lead the charge. Remember Kael's AOE spells can damage buildings, and have Alessandra use her Q on Dar'Khan or the odd Frost Wyrm in the base: the AI is bad about breaking her channel and she can get permanent or near-permanent uptime on her lockdown.
Advice blurbs for the other missions:
Ch2 - Lean heavily on the naga. Churn out their upgrades quick and mass Myrmidons. There's no need to rush down the power generation and the naga never get attacked. Move quickly to take the neutral clans, and mass Tidal Defenders at the T-junction directly between yours and Illidan's base.
Ch3 - Abuse Aewynne's stealth to reach the Mana Domes without killing too many enemies. 50 blood elves is really generous, and the side quest ends when you destroy all 3 domes. Save Alessandra's Q for Liadrin, Blood Knights (they look like Night Elf Sentries), and Dragonhawk Riders in that order. When you get to the last Liadrin fight at Tempest Keep, sneak units along the right side up the ramp and poke at her from long range, then pull her down the ramp and take her down there. She should come down with very few of her allies, making it easy to focus her down. If you can get to the defend portion with most of your units healthy and your gold supply good, you should be fine. Abuse the high ground.
Ch4 - Mass towers, especially Arcane Towers, at your north entrance. That'll easily repel Dar'Khan. The enemy never attacks your other entrances aside from the scripted attack at the beginning. Bring a full control group + ballista to Lor'Themar, then take all his combat units and yours and take out the purple Scourge base, it should be easy. Remember to use Sorceresses to silence the miniboss Necromancers.