Something you can lag bolt to the studs in a closet. And then putty up the holes when you leave.
It not being your property leads to some issues, you just might have to kiss some of your security deposit good-bye. A full duty, full size safe when filled may easily overload the structure and if you do that, your security deposit may be the least of your worries if you willfully damage the structure.
I've seen big eyebolts into studs in the corner of a closet with a big bike U-lock through trigger guards. The guns prevent the eyebolt from being turned out, and it can be damn hard to pull the eyebolt out.
If you are willing to putty and paint on leaving. and you have the right configuration, Get bar stock at your local hardware store, drill AND tap holes in the bar stock to line up with mounting holes on safe or gun vault. Drill through holes in the wall preferable straddling a stud. Bolt the safe through the walls to the bar stock, with the heads of the bolts in the safe. That aint gonna go anywhere easy until you get the safe open, and if they can get the safe open, they aren't going to take an empty safe.
And when I was in an apartment, most of my guns were back home (unlocked) and those in the apartment were in ready service locations, not locked up. Danby was at sea and Ma Danby knew where to find and how to use the 1911. (The bastards stole my International Scout, but it wasn't in the apartment.)