Killing Cows helps a lot for money issues as well as training., collect their hides and sell them at GE for around 116Gp ea 1 inventory full = 3,248 Gp, don't sound a lot but it stacks.
You could depending on what specific skills you have trained.
You could kill Deadly Red Spiders, go to the 2nd level in the stronghold of security and kill zombies.
Hill Giants will be best for you to train on, they drop big bones, collect those to earn a bit of cash whilst training.
Recommending monsters to kill is impossible without knowing your primary goal for killing (such as for experience or money) and your combat level. There are thousands of possible monsters to kill.
Its possible to kill a couple of green dragons at your level but could be very difficult to do. I'd recommend getting a higher combat level like in the 70s or 80s. Not difficult to do and it will pay off once you get to green drags. :)
The only best way to raise a combat stat is to kill Lesser Demons at Karambja Volcano. They are level 82.
bronze dragons
At your level, I would say killing Experiments ( Level 25 - 100Hp = 400exp per kill ) near Canfis.
train kill stuff
In order to get pure defence in Runescape, you kill monsters using defence stance accessed in the combat style thing. That is how you get pure defence in Runescape.
You can go into the wilderness and fight dragons, don't forget the dragon-fire shield, but be careful of revenants they can still kill you easy.
The Reason Is Because If Everyone Was A High Level That wouldn't be fun would it? Also if every person was level 138 it would be way easier to kill dragons ect. Your Welcome :)