Without arms, they'd just have to burrow into something. They can probably dig similarly to a stuck tire, using abrasion to burrow into a hillside or the like. With only a family of one, they just need a roof.
The closes thing to a ideal mecha today, is a battle helicopter with optional legs.
So if the sneaking around mountain-tops does no longer work out. It can "run/climb" into denied areas, take a shot and then retreat.
Walking should be a measure of last resort.
So how would it look? Like a shitty compromise always look- not very sleek but functional....
There were advanced mecha (humanoid robots; about the size of a person and remotely controlled) in a book called Gamer Army. Since bullets could (and did) hurt these mechs, and missiles would do the same (but worse), why would these mechs be viable in combat?
For the same reasons already posted, and a few more:
Agility-Tanks aren't good on truly rough ...