1:) NEVER put any units behind the Slayers! They're only intended to hold a unit for a short time, so the enemy unit has a good chance of fighting anything behind the slayers. A better tactic is to use another unit to flank the enemy while they're engaged with the Slayers.

The unfortunate way I learned was when the Slayers encountered Scott's Night Goblin regiment. The 3 Fanatics in the unit promptly sprung out, killed 6 Slayers and then proceeded to pop out the back of the unit and attack the warriors, who in turn, took 5 casualties. Ow. Fortunately the fanatic didn't do anything else for the entire battle. The Slayers managed to avenge their 6 brethren by taking out 7 night goblins in one sweep before enjoying a glorious death.

2:)Protect your Thunderers as much as you can! They have very little defense in hand-to-hand combat and are therefore vulnerable to flank attacks. I've attached shields in this case, which helped a bit.

My Thunderers, after receiving heavy losses due to the goblin's Doomdiver, proceeded to be charged by an Orc Chariot, which had managed to escape the clutches of my Ironbreakers and attack me on the flank. The Chariot later fled, and did not attempt to charge my flame cannon because of the rough terrain of the hill (it would have been severly damaged), but I lost all my Thunderers in the ensuing battle. This left me without a great unit - they had been effectively whittling away at the night goblin and Big Un's units.

My flame cannon, as always did a great job, despite encountering its first misfire! Luckily, it was only unavailable for two turns and got right back into blowing away as many night goblins and Orc Boar Boyz as it could :) All in all, I'd have to say that this unit is one of the most important and reliable in my army.

My Ironbreakers and Longbeards stuck it out to the end as always and only suffered minor casualties. These units are incredibly good at taking impact hits from fast cavalry and chariots that happen to try and flank charge.