the probably fastest way to lvl up in that lvl range is to take the room in hl1 above the hunters room which has Golems and alot of other ice Monsters.
take capes with you and place yourself in the middle of the room so the cape actually lures all or atleast most of the Monsters.
we got a really nice tool here which is the channel changer.
this allows you to use cape, kill the room and instead of waiting for the respawn you simply change from CH1 to CH2 and do the same, then you change to CH3 of course and then back to CH1.
like that you will always have a full room when you are ready to cape which gives you a very decent lvling speed.
Where are this spot in HL 1?