I must say Marcus is my favorite master in the whole game, but I do seem to have more trouble playing him over my other Masters, he just has this extra hidden level of finesse that I haven't quite mastered yet.
but 3 of my Biggest recommendations for you are:
1) I don't use The rattler and Cerberus in the same list, they both are a bit too high cost for what they do but its not an issue to use one of the other
2) try out a Shikome, since it is a beast you can hire it, and it's soooooo good at taking down key models. While it can only target one model until that model is removed from the board it gets a huuuge bonus against that model so its always good for taking out kep models, just be sure to remember you need to not over extend it, other wise it will die
3) back to 1 with the Silurids and Night terrors, both are such good models and I love them to death, but every time I've ran night terrors and silurids in the same list one has done spectacular while the other group did absolutely nothing and I wished I would have brought something to replace them.
other than that, some really cool models to take are Kaeris or VonSchill because they are such beasts in the game