I play Marcus quite a bit, and I will give you my 2 cents.
First read the stickied post on Marcus to make sure you have general knowledge of all your choices, a lot of the early stuff is a bit older now, but there are some good nuggets on strategy and tactics in there.
#1 recommendation would be Molemen. They are not sexy like siluruds but for 1 point more, you can have 2 of them. They are semi durable, but are surprisingly speedy with a little effort. I usually leap a model forward (Blessed of December usually, sometimes something else), and leave a scheme marker within 8 inches of a moleman. Then the moleman can use his burrow to move to touching that marker, and it walks once and sets down a marker. Next moleman burrows and then walks and leaves a marker, or just moves twice to get past the first. then you can start leapfrogging around, and if they send something after the moleman, you can burrow out of engagement, and keep skipping around. You can also leapfrog with just two of them. Also, leaving all those markers all the time will smokescreen schemes sometimes.
Another Beast that can be underrated are the Hoarcat Pride. The punch way above their weight-class while unhurt, unfortunately they are not the most mobile, but can move through other models, so you can screen them with waldgeists or bigger model and then have those little kitties charge through.
I almost always run Marcus with his own upgrades. If you are very aggressive (as I am) you will like Trail of the gods, give yourself 8 inch walk, +2 damage or even an additional ML attack, first one free, a tome discarding for each additional. This helps Marcus get to where he needs to, but also turns him into a min 4 damage model with up to 4 attacks if need me. I also combine it with the hunger cry and feral instincts usually. FI lets you turn someone into a beast ( and do 2 different 0's, and Marcus has good ones) and the Hunger cry can help you boost beast wp and make all enemy beasts in LOS take a negative twist on duels! Combine that with Raptors and Remains turning enemies into beasts, and suddenly any of them in LOS of Marcus will be at a negative twist modifier (more importantly, if they end at a negative twist or worse, no cheating!) thus easy meat for Beast beatdowns.
Everything else people say is nice, but I find it wise to think about imbued energies for both Myranda (4 cards when she sacrifices to become a new beast) and often one for Sabertooth Cerberus to get fast for an alpha strike.
One mean little trick you can consider is make sure you outactivate your opponent (there are lots of nice cheap beasts, so this is not hard against non-gremlins/Hamelin) and use Darzee's Chaunt on your Sabertooth Cerberus (this gives a positive twist on attack and damage, combined with 3 headed, that is 2 positive twists on his attacks and damage, and if you have outactivated the double negative to defense is no big deal), then alpha him. He can discard imbued to become fast, leaps in, charges, (about a threat range of 20 with one walk) beats the snot out of something (hope or have Tomes, so you can Maul trigger for additional attacks), then activate the SC again, charge something else or just attack something if range, either choosing to leap deeper for an additional 15 inch range or save that leap so you can leap into cover or away from danger when done attacking.