Both board dominance and assassination plays can work, you just need to make sure that you're doing things to efficiently support whichever play style you're implementing, and be flexible enough to switch if necessary. There comes a point in any board dominance play where you need to focus down the enemy mage.
If I'm in a good spot to go after the opposing mage, I'll often do that. Tanglevine is generally the best/cheapest way of bypassing powerful guards with multiple creatures, while Elusive is the best way of bypassing guards for a single creature.
If I don't think I'll be able to kill my opponent without dealing with some of their creatures, I will try to find a way to build an advantageous board position. I've had games where I kind of ran out of steam because I didn't worry enough about board dominance, and let my opponent build up a huge creature advantage. You can still win from this position if you've done enough damage already, but it's a precarious position for most mages to be in.
I've killed Iron Golems before, but it is fairly resource intensive, and it's generally better to work around them. Iron Golems are hands down the most efficient creatures in the game from the standpoint of a pure slug fest, which is why they make such phenomenal guards. They are just so tough for their cost.
Earth Elemental is significantly more expensive, and isn't an effective guard, so it's more difficult to promote a slug fest with them. I almost killed an Earth Elemental once, but my opponent conceded before I could finish it off. Mostly I was trying to keep it Stun locked with Temple of Light before the nerf.
Bridge Troll is very weak against multiple threat sources without Rhino Hide, but the advantage of Rhino Hide is that it only costs 1 mana to enchant Bridge Troll if you're using Enchanter's Ring, and you don't need to reveal it until armor will benefit you. The other issue with him is that fire attacks will kill him incredibly quickly. This can be an issue against Lord of Fire, the Warlock, and anyone with a copy of Fireblast. I still like Bridge Troll, but I don't think he fits into my current builds very well. Without armor, Bridge Troll is too easy to focus down. With armor, he's more of an investment.
I've actually been working on de-emphasizing fire damage in my Warlock book, since Dragonscale Hauberk and Elemental Cloak are so efficient at defending against it. I still use Fireball and Fireblast, but I've stopped using Lash of Hellfire and Lord of Fire. I'm thinking about adding a Hurl Boulder, but those are all currently tied up in other spellbooks. I can probably pry one loose from my Earth Wizard build though.
Valshalla and Samandriel are both relatively vulnerable to large attacks for their cost (same with the wind angel, but she just seems much worse overall). You really need to give them an extra armor or 2 if you don't want them to be focused down. This doesn't matter if your opponent never attacks your creatures unless they're guarding, but it can be an important factor if your opponent goes for board dominance.
