The Withered Army Training scenario is a great way to get some extra Nightfallen rep, toys, pets and various other goodies. Some people are struggling with it though so I'm aiming to help a few people out who might need it.
The first thing I suggest is to go the distance and pay up the full 2000 (or 1300 if you aren't honored) mana cost. The scenario only resets every 3 days so it's worth the extra bit of time investment. The second thing is to use a tank spec, if your class has access to one. Not only can you protect your withers from melee damage, but you often gain access to plentiful stuns and interupts.
Thirdly, don't open any chests until the end, it consumes your withered and you want to hold as many as possible to get through the bosses.
The first room of the scenario introduces you to the basic Withered Shambler mobs. These guys do very little but can sometimes fixate a withered and beat on him a bit. It's important to watch out for this and protect them accordingly. The real meat of the first room is the Volatile Wraith. These are the ones that are going to cause problems for newcomers to this scenario. They have a 2.5s cast time skill called Arcane Munition, this is going to cause an explosion at your location and deal heavy damage to your withered. Interrupt this at all costs. Unfortunately he casts it more often than the interrupt skills cooldown, however you can run out of line of sight to delay him. At 25% he attempts an Unstable Explosion. This will deal heavy damage and a knockback. This is where stuns come into play, stun him and have your withers nuke him down before he can cast again.
Truthfully none of the other enemies are too dangerous, so I'll skip going over every single one and just give a bit of a rundown of the bosses and how to deal with them. Worth noting that every boss is stunnable. Additonally a good technique to practice is to clear boss rooms before engaging a boss, thus allowing you to use the withered frenzy mode safely.
Lapillia – The first boss you usually run into, locked behind a door that requires 10 withered to open. She has a cast time bolt attack that deals single target damage but is interruptable. The real problem is the Crystalline Shrapnel. She'll throw a debuff onto you and at the end of the duration you will damage all of the withered close to you. It's important to run far away as the debuff expires because the effect is bigger than the animation would suggest.
Leystalker Dro – Arguable the toughest boss in the scenario. He'll ambush you from stealth at some point during the Falanaar Tunnels section, so it's important to pull slowly and carefully here so not to be caught off guard. Really the most you can do with this guy is just hope to kill him really quickly. He's going to charge around and slash withered for heavy damage, as well as buffing himself with an attack speed buff and slamming you for massive damage too. Stun and engage frenzy mode to deal with this guy swiftly.
Brood Guardian Phyx – Trivially easy boss. He'll try to summon in extra spiderlings with his Brood Expansion cast, just stun this to minimise the number that spawn, and pick them up to protect your withered. He boasts a frontal cleave attack called Cleaving Claws, however you can interrupt this and he doesn't use it very often. Should have no trouble here if you managed to pass Leystalker Dro with minimal losses.
Volatile Wraithlord – This guy is an exact clone of the regular Volatile Wraith, except with more HP. Deal with him in the exact same way as the other mob.
Furog the Elfbreaker – As the name suggests, this guy will break your elfs. His room is full of constantly spawning spiderlings, so a good technique is to kite him back into the hallway so you can use frenzy mode without being disturbed by the spiders. He has 2 abilities, both of which can be stunned but neither can be interrupted. The first is Grab Withered, he'll pick up one of your withered and deal damage over time to them, at the end of the duration he'll slam them into the ground, usually instantly killing them. It's tempting to try and stun this but saving the stun for his 2nd ability is more important. Either eat the loss of 1 troop, or use external cooldowns on the troop to save him (Blessing of Protection works well). The more dangerous ability is Stomp. This one deals heavy damage in a large area around him and knocks everything back. You absolutely don't want your withered taking this damage, so stun him to prevent this one.
Past this point you will be assaulted constantly by small spiderlings until pulling the final boss.
Psillych – An easy final boss compared to the Elfbreaker. Casts a web blast which leaves webs on the ground, damaging and slowing anything caught in them. You can stun this, however he casts it reasonably frequently so the chances are some withered will be caught in it. He doesn't do anything special besides boasting a large health pool. If you manage to make it this far you can probably beat him easily.
If you're interested in more detail, I have a video with a complete walkthrough of the scenario, detailing every section. https://www.youtube.com/watch?v=upHYIokrsg8