Warcraft 3 Frozen Throne Walkthrough for Night Elf Sentinels:
Chapter 2: The Broken Isles
I find building a base preferable to squad missions any day. You start with a pair of wisps that should be transformed into moon wells to raise your population cap. Set all your wells to auto-cast their heal spell so that you can move your troops in range to get refreshed. Make six or so more archers, a pair of huntresses and a glaive or two. You will likely need more Moon Wells.
Main Quest: Locate Illidan
Wait until the Naga attack for the first time to head with Maiev to do some creeping to the east. A decently sized group with archers and sentinels should be added to Maiev's party, while your Ancient of War keeps pumping out an army. You should upgrade your tree as soon as you can, as well to make an ancient of lore and druids of the claw to accompany Maiev. Druids of the claw are great. They do good damage themselves and can heal your other units. Once you have a full army (sentinels and archers) head to the east to kill the behemoth. Once it's down, you can make Maiev blink to the landing to the northeast to get a pendant of energy. Head back through your base and heal your units at the moon wells, then proceed west and kill all the Mur'gul in the base that snakes to the south. You can find a Maul of Strength, not exactly the best item for Maiev but it works for now. You can grab scrolls of healing at the Goblin Merchant here.
On north you'll meet a former Orc Warlock who gives you a new quest.
Optional Quest: Silence the Ghost
Head back to your base, which should be booming and good to go with a nice army to accompany Maiev on her quest. I'd recommend four ballistas in their own group, with three druids of the claw, four huntresses, and four archers accompanying Maiev. Load your army into the boats (leave some behind -- remember the Naga are aquatic) and venture where you can find a hydra on an island. It drops a periaph of vitality, which increases Maiev's hit points by 150.
Make your way now to the optional quest location. Focus on getting to the pits and only attack the skeletons if you need to defend your army. Once all three pits are destroyed, use the boats to land on the northern side of the island and talk to Drak'thul to receive a robe of the magi +6. Maiev should have almost 600 mana now. Heal up your army before proceeding, and buy an extra boat to send by itself back to your base to grab reinforcements later. Work on a full complement of archers to go with your ballistas' hotkey group. and get them in this ship.
Move on to the western island and work on obliterating the Naga. It doesn't matter much which side you land on to take out the red Naga base, but I prefer the northern section. Focus your ballistas on the static defenses, while your army focuses fire on the Naga that guard them. It shouldn't be hard to win without a larger army, you just need to learn to be quick with druids' rejuvenation spell. Dropping a healing ward in the middle of your army during the fight will help to slow losses, also.
If you go north on this island, which you should do with a transport ship, there are a pair of enraged jungle stalkers that will drop a claws of attack +6 for Maiev. Use your druids to heal up once more before jumping back in the ships to attack the final Naga base. You should have more than enough firepower to end the fight quickly, and reinforcements are not far away. Set your buildings' waypoints to the northern landing of your starting isle and ferry them across.
If you need any more reinforcements, bring them along then go through the gate to the northeast. There are actually not that many Naga guarding the tomb, so the final fight for this chapter is easy.