Kill things!! Lol
It depends on your level. I'm level 57 and I find fighting Ice Giants and Moss Giants to be quite fast.
I reccomend killing Yaks at Neitiznot (Requires start of Fremennik Isles quest) which are level 22, hence having a max hit of 1, but have 50hp, allowing you to hit hard upon them.
Also, if you have gotten to a certain point in the Creature of Fenkenstrain quest, the level 25 "Man-Spider" and "Wolf-Sheep" Experiments also have a max hit of 1 with 100hp, allowing you to hit MASSIVELY upon them.