ill post this as it is related to this topic:
Lets explain how attack attribute works. Attack attribute can give up to 24% dmg boost on an attack.
If for example a mob has Earth Resist rating of 100, and your attack attribute for Earth is 100, you will get no dmg boost on an attack. If your attack attribute is less than 100, you still get no bonus but there is also no negative effect from this. If your attack attribute is 200 in this situation, you will get a 10% dmg bonus on an attack. The max element you can have in this situation to impact your dmg is 340 attack attribute. At 340, you will get 24% dmg boost on an attack. If you have 350 attack attribute, you will still only get 24% dmg boost. There is no benefit going above the upper limit.
Now, the question to how much element resist a mob has. See table below. After 118, things may cap out. More testing on this update is needed.
110 - Mob Resist = 1080 / Attack Attribute must be 1080-1320
111 - Mob Resist = 1180 / Attack Attribute must be 1180-1420
112 - Mob Resist = 1280 / Attack Attribute must be 1280-1520
113 - Mob Resist = 1380 / Attack Attribute must be 1380-1620
114 - Mob Resist = 1480 / Attack Attribute must be 1480-1720
115 - Mob Resist = 1580 / Attack Attribute must be 1580-1820
116 - Mob Resist = 1680 / Attack Attribute must be 1680-1920
117 - Mob Resist = 1780 / Attack Attribute must be 1780-2020
118 - Mob Resist = 1880 / Attack Attribute must be 1880-2120 *there may be an upper limit cap to element on a mob
119 - Mob Resist = 1980 / Attack Attribute must be 1980-2220 *there may be an upper limit cap to element on a mob
120 - Mob Resist = 2080 / Attack Attribute must be 2080-2320 *there may be an upper limit cap to element on a mob
This should serve as a nice baseline on what to take/use.
Also, in my personal experience, mob attack rating is very similar to these ratings as well. 1800-1900 earth resist on defense is a noticeable benefit in places like Wall of Argos, etc.
edit: some ppl say the element increases on even levels only, moving up in 200 chunks (ex: 110, 112, 114, etc). If you follow that guideline, default to the previous level rating for odd levels (111, 113, etc)