"But apart from increase each monsters life by 10"
This is the answer but they wouldn't have to multiply the life by 10.
The monsters keep getting more and more powerful attacks but the monsters' life (other than a few exceptions) hasn't gone up as much. This has resulted in many monsters one hit=one kill kind of scenario.
Give monsters more life and/or increase the power of health runes and battles will last longer.
This will help to balance out the value of runes as well. Right now, few people value health runes. If health runes turn monsters into tanks, more people will use them over the speed and power runes.
It would be interesting to see if people would start to use health runes more if they were all doubled in power?