The battle rating, which is currently calculated as ATK + DEF + sqrt(HP) + CS is misleading. I have done some experiences. Here are  the results I have found. The stats are given in the following order: HP/ATK/DEF/CS/displayed battle rating

combat 1 : sovereign (40/10/12/1.3/29) versus spider (30/7/0/1/13). Sovereign victorious on turn 5 with 32 HP left

combat 2:  sovereign (32/10/12/1.3/28) versus bandit (10/5/0/1.3/18) and eben wolf (250/4/0/1/9). Sovereign victorious on turn 4 with 32 HP left

combat 3: sovereign (40/10/12/1.3/29) versus NPC group of 3 composed of Raeguile(33/1.5/4.2/1/38), Lilokhi (33/3/0/1.3/9) and Norak (40/10/0/1.3/17). Sovereign victorious on turn 18 with 25 HP left.

combat 4 : sovereign (40/10/12/1.3/29) versus Troll (241/20/0/1/36). Troll victorious on turn 5 with 194 HP left

combat 5 : sovereign (36/10/12/3.3/31) versus Troll (241/20/0/1/36). Troll victorious on turn 7 with 163 HP left

combat 6 : sovereign (40/10/8/3.3/29) versus Troll (241/20/0/1/36). Troll victorious on turn 4 with 189 HP left

combat 7 : sovereign (40/12/12/4.8/35) versus Troll (241/20/0/1/36). Troll victorious on turn 6 with 92 HP left

combat 8 : sovereign (40/12/12/4.8/35) versus ennemy sovereign (40/10/4/1.3/21). Sovereign victorious on turn 2 with 40 HP left

The battle rating of the troll doesn't give a sufficient sense of danger. This is mainly due to the large HP that the troll has. Defense definitely help against ennemy with few ATK. It also helps to survive longer .

The combat speed allows to strike lots of hits during a turn. At 1, you attack 2 times in a turn, a 4.8 you attack 5 times in a turn, which increase the odd to succeed against trolls.

After playing with some formulas, I have found some interesting results with the following one:

HP + ( ATK + DEF) * round ( CS )

Using this formula, I got:

combat 1 : 62 against 37 (Sovereign victorious on turn 5 with loosing 8 HP )

combat 2 : 54 against 15 and 24 (Sovereign victorious on turn 4 with loosing 0 HP )

combat 3: 62 against 38, 36 and 50 (Sovereign victorious on turn 18 with loosing 15 HP )

combat 4: 62 against 261 (Troll victorious on turn 5 with loosing 47HP )

combat 5: 102 against 261 (Troll victorious on turn 7 with loosing 78HP )

combat 6: 94 against 261 (Troll victorious on turn 4 with loosing 52 HP )

combat 7: 160 against 261 (Troll victorious on turn 6 with loosing 149 HP )

combat 8: 160 against 54 (Sovereign victorious on turn 2 with loosing 0 HP )

Sure this formula isn't perfect but it seems to give a better feeling of the outcome