The only thing that can really kill us on this level are the myrks.  So be sure that the lock has enough mana to cast his confusion spell.  The level begins with a showdown with Shiver.  If any unit marches past the Deceiver they probably will be killed by Shiver.  Therefore Myrdred should march towards Shiver.  Shiver will retreat once the Deceiver comes forward.  After Shiver leaves the picture, we can work our way to Shiver's camp.  The souls up ahead should be taken out by the lock.  Myrdred and the heron should distract the souls by running back and forth.  These units should be careful not to get in the lock's line of fire though.  It's best we stay out of the moutain ridge passes, it's too constricted with nowhere to go if things get hairy.  Soon the thrall will start streaming towards us.
The heron and Myrdred should stay back while the lock, dorf, and archer work.  The thrall will come at us in almost a single file line.  The lock can maximize his destructive capacity if instead of firing at the nearest thrall, aiming just to the side of the single file line.  Try to get the fireball to explode right next to the single file line.  The main target for the lock should be the middle of the line.  When the myrks show up the Deceiver should wait until the lock has confused the myrks until he converts his myrk.  Obviously it's desirable if he can convert a myrk that escaped the locks confusion spell.  The lock should wait to confuse the myrks until they are very close.  Once the myrks are running around all confused the melee should rush forward to kill them.  The dorf should check his fire, it's better if the melee kill them because the Deceiver may have converted one of the confused myrks.  It's also better if the melee get them because otherwise the heron get to do almost nothing!  If the group runs into myrks when neither the lock nor the Deceiver have enough mana to cast their spells, then the team should retreat.  Hopefully the mana will be regenerated in time for us to confuse or convert them.  I am not a big fan of the secret artifact thingy.  I think the confusion spell is much more effective.  However the artifact is very handy if the dorf is dead at this point because there is a huge patrol of mryks right at the end. 
         The giant myrks should be dealt with like this:  The Deceiver should march forward and get them to throw their rocks at him.  The giant will then chase Myrdred.  If it's just a single giant, then Myrdred should turn and use his staff on him.  The melee should then rush forward and help Myrdred.  Once one of the myrk giants is dead, a huge mess of smaller myrks will come running from Shiver's camp.  Hopefully the lock has enough mana to cast confusion on them.  If he doesn't.......we're in deep d'oh.  After the lock confuses the horde of myrks, the dorf should bomb them at will.  Again if the dorf is dead the lock artifact is handy against this mess of mryks.  The pack of three giant myrks can be dealt with just like the single giant was dealt with except for one important difference.  The lock needs to confuse them once they throw their rocks at Myrdred.  You can also just sneak past the giant myrks if you want. 
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