If they go for garrisons, rush a 7.5cm, if they are out in the open, try to flank with 2-3 squads. One comes in from the side, one from the front and one at another odd angle. At least one should make it, or send a kubel around the side if it's the first maxim.
Opening something like: Starting Sturm.
Sturm
Kubel.
Every time I've been against maxims with this strat I've done well, except on maps like Ettelbruck where its hard to flank due to the very narrow chokes that maxims do great in, if its 3v3 it's fine, 4v4 on maps like that are just cluster fucks and you can't flank reliably.
Rushing Obers are good too, Obers with good RNG will kill the gunner of the maxim with the first burst of their MG. so if you have 2 or a flank squad, you should be fine.
I generally go Mechanised truck, convert to fuel, get Flak truck next in a decent position and either go Luchs if its heavy infantry, or straight for a Panther while getting 1-2 ober squads to support my Sturm/volks. - Don't forget Rocket43's for some early-mid Anti Tank while waiting for Panther
