Hey, i am a Zerk and run FK H5 daily, somethings that i noted on my runs :I'll definitely take those tips into account, something I always find myself stuck in is the frost wind at last stage. Basically stuck doing h3 til I gear up more, such a struggle to get something that's not magic stats :')
1) To run FK Heroics I, II and III, you need some gear with good stats, some seals are a bonus and make your life easier (a good ROM Set +10 works here);
2) To run FK H4-H5 you NEED good gear +10, with good stats and good seals or you will die on each encounter, elites included (a very good ROM Set+10, with good seals is necessary or get a FK parts with some Corrupt Set parts for bonus and put it on +10, with good Seals and a Pet);
3) To do FK H4-H5, good gear is essential for all the classes, think on FK as a team, tanks need a 18k hp pool, assassins need a 5k dps at least, so you too need a good gear to be part of the team;
4) A good tip is let the tank "realy" take the aggro before you enter in a fight, give him time to do it;
5) Try run the FK H5 with same people, over and over again, it realy help in the work of positioning as a Zerk...
Now, some tips for the Boss Encounters:
1) First Boss - Stay always behind him to not take the damage of normal blows and whem he cast the "Rage Hammer", use the defense "skill block with sword", it gives you a 2sec invulnerability and so imune to the efect of rage hammer;
2) Second Boss - Mage/Yeti - Let tank have the agroo for 5 or 10 seconds on both, them you can just focus on Mage, just try NOT stay in front of yeti, because some of the skills, like the uppercut or the ice flow, even if not directly target on you, can kill because of aoe;
3) First encounter with Rondo is the easy part, not much to say here, stay behind him, use defense (2sec invulnerability) when he does the atack of multiple blades wich i dont remember the name and move for positioning when needed;
4) Second and last encounter with Rondo, positioning is essential here, zerk and assassin are made of paper for Rondo, even with very good gear sometimes Rondo give hit kill on criticals or if you get caught on blizzard skill, you are dead, so is important:
* Stay behind him is a must, is much better move away and slow down the DPS a bit, over be a dead DPS;
* Use combos and charged skills with caution, only use multiple combos or super charged skills, when he gets destabilized or when he uses the Frost Path skill;
* Zerk do not have a escape move, so use your charge when need a fast exit move, to do it, target an ally (mage or healer) and charge on him, moving out of zone of danger;
That 's all I can tell so far, hope it works for you and helps on becoming runs!
P.S: Sorry for my english, its not my first language...