man, i told that shyvana is a good tank. if ur team hasn't a tank, u cannot play shyvana ad, besides this build is for those who want to buy tank items. If u want to buy some attack damage and speed, buy trinity force and black cleaver insteed of sunfire and force of nature.

Guys, i said 3,7k hp, because i've got full rune page for +300hp at lvl 18

Too much beef, not enough damage.
Shyv has no built in cc, so I like the mallet, but I don't think that's enough to make her a threat in team fights. As the build stands right now, if you focus on the enemy carry to scare them away from AAing your team a new nose hole, you'll get enough damage to accomplish that with your very last item.
Also the zerker's are a bit out of place. Merc treads would fit the tanky style better, also she really needs the ccr, being a melee fighter and all.
If you switch the boots to merc treads, it would fill in the MR gap you've got.
If you switch the atma's in to the build earlier, it would keep you at enough damage to be a threat through mid game. Maybe try out phage->sunfire->atma's->frozen mallet. With her q proccing on-hit effects, you can get phage fairly consistently if you hit it after an AA.
Last 2 things.
I'm not a fan of the runes, but I don't tend to think they make that much of a difference, and they're often suited to your playstyle.
Also, you should prolly take GA out of the main build and put in a situational item section instead. You already say you can instead build trinity force, but I'd say go further than that. One item that jumps to the front of my head is Last Whisper. Having no flat armor pen, LW and her E would add up REALLY fast and increase your damage output a lot.
um... sorry for the wall of text...