A brief explanation of the tables below. The first 6 values (252) represent the EV's, one value for each stats.
The next 6 values (31) follow the same pattern as the EV's, except that these values are from the IV's. 

After the values of the IV's comes the number that will define the standard ability of the table. If you leave it at 0 this will set the
table to have the Pokémon's 1st ability as standard; if you define 1 to 2 ° ability of the Pokémon it will be the default of the table.

The next value is 25. This value represents the nature index, there are 24 natures in all.
Setting it to the number 25 will ensure that the default nature of the table will always be random.
If you want to configure a specific nature for a given table, just change the 25 to the corresponding number
of nature. 

Here is a list of the ID of each nature: 
0 HARDY  || 1 LONELY || 2 BRAVE    || 3 ADAMANT || 4 NAUGHTY  || 5 BOLD   || 6 DOCILE || 7 RELAXED
8 IMPISH || 9 LAX    || 10 TIMID   || 11 HASTY  || 12 SERIOUS || 13 JOLLY || 14 NAIVE || 15 MODEST
16 MILD  || 17 QUIET || 18 BASHFUL || 19 RASH   || 20 CALM    || 21 GENTLE|| 22 SASSY || 23 CAREFUL
24 QUIRKY|| 25 RANDOM NATURE

List of Balls and their index: 
1 Master Ball, 2 Ultra Ball, 3 Great Ball, 4 Poké Ball, 5 Safari Ball, 6 Net Ball,
7 Dive Ball, 8 Nest Ball, 9 Repeat Ball, 10 Timer Ball, 11 Luxury Ball e 12 Premier Ball.

NOTE¹: In HxD you must put the number in hexadecimal, that is, instead of by 10, you must put 0A.
Thus: 
01 = 01  || 02 = 02  || 03 = 03  || 04 = 04  || 05 = 05  || 06 = 06  || 07 = 07  || 08 = 08  || 09 = 09  || 10 = 0A
11 = 0B  || 12 = 0C  || 13 = 0D  || 14 = 0E  || 15 = 0F  || 16 = 10  || 17 = 11  || 18 = 12  || 19 = 13  || 20 = 14
21 = 15  || 22 = 16  || 23 = 17  || 24 = 18  || 25 = 19

If you want to create a table with nature Adamant as standard, for example, just change the 25 (19 in Hex) for 3.
I already left these EVs / IV's tables pre-configured.
Follow the list: 

Index 00, 252 EVs, 31 IVs, 1° Ability, Nature Random, Great Ball.
Index 01, 252 EVs, 31 IVs, 2° Ability, Nature Random, Great Ball.
Index 02, 252 EVs, 31 IVs, 1° Ability, Adamant, Master Ball.
Index 03, 252 EVs, 31 IVs, 2° Ability, Adamant, Ultra Ball.
Index 04, 252 EVs, 31 IVs, 1° Ability, Bold, Master Ball.
Index 05, 252 EVs, 31 IVs, 2° Ability, Bold, Ultra Ball.
Index 06, 252 EVs, 31 IVs, 1° Ability, Brave, Master Ball.
Index 07, 252 EVs, 31 IVs, 2° Ability, Brave, Ultra Ball.
Index 08, 252 EVs, 31 IVs, 1° Ability, Calm, Master Ball.
Index 09, 252 EVs, 31 IVs, 2° Ability, Calm, Ultra Ball.
Index 10, 252 EVs, 31 IVs, 1° Ability, Careful, Master Ball.
Index 11, 252 EVs, 31 IVs, 2° Ability, Careful, Ultra Ball.
Index 12, 252 EVs, 31 IVs, 1° Ability, Gentle, Master Ball.
Index 13, 252 EVs, 31 IVs, 2° Ability, Gentle, Ultra Ball.
Index 14, 252 EVs, 31 IVs, 1° Ability, Hasty, Master Ball.
Index 15, 252 EVs, 31 IVs, 2° Ability, Hasty, Ultra Ball.
Index 16, 252 EVs, 31 IVs, 1° Ability, Impish, Master Ball.
Index 17, 252 EVs, 31 IVs, 2° Ability, Impish, Ultra Ball.
Index 18, 252 EVs, 31 IVs, 1° Ability, Jolly, Master Ball.
Index 19, 252 EVs, 31 IVs, 2° Ability, Jolly, Ultra Ball.
Index 20, 252 EVs, 31 IVs, 1° Ability, Lax, Master Ball.
Index 21, 252 EVs, 31 IVs, 2° Ability, Lax, Ultra Ball.
Index 22, 252 EVs, 31 IVs, 1° Ability, Lonely, Master Ball.
Index 23, 252 EVs, 31 IVs, 2° Ability, Lonely, Ultra Ball.
Index 24, 252 EVs, 31 IVs, 1° Ability, Mild, Master Ball.
Index 25, 252 EVs, 31 IVs, 2° Ability, Mild, Ultra Ball.
Index 26, 252 EVs, 31 IVs, 1° Ability, Modest, Master Ball.
Index 27, 252 EVs, 31 IVs, 2° Ability, Modest, Ultra Ball.
Index 28, 252 EVs, 31 IVs, 1° Ability, Naive, Master Ball.
Index 29, 252 EVs, 31 IVs, 2° Ability, Naive, Ultra Ball.
Index 30, 252 EVs, 31 IVs, 1° Ability, Naughty, Master Ball.
Index 31, 252 EVs, 31 IVs, 2° Ability, Naughty, Ultra Ball.
Index 32, 252 EVs, 31 IVs, 1° Ability, Quiet, Master Ball.
Index 33, 252 EVs, 31 IVs, 2° Ability, Quiet, Ultra Ball.
Index 34, 252 EVs, 31 IVs, 1° Ability, Rash, Master Ball.
Index 35, 252 EVs, 31 IVs, 2° Ability, Rash, Ultra Ball.
Index 36, 252 EVs, 31 IVs, 1° Ability, Relaxed, Master Ball.
Index 37, 252 EVs, 31 IVs, 2° Ability, Relaxed, Ultra Ball.
Index 38, 252 EVs, 31 IVs, 1° Ability, Sassy, Master Ball.
Index 39, 252 EVs, 31 IVs, 2° Ability, Sassy, Ultra Ball.
Index 40, 252 EVs, 31 IVs, 1° Ability, Timid, Master Ball.
Index 41, 252 EVs, 31 IVs, 2° Ability, Timid, Ultra Ball.

NOTE²: There are, in total, 255 customizable tables like this one, I configured only the first 42, so there are still 213 tables left
for customization, if you wish.
Each table is 10 bytes in size (16 in decimal).
Below is an example below table 0: 
*FC FC FC FC FC FC*-*1F 1F 1F 1F 1F 1F*--*00*-----*19*-----*03*---00
-Ev's----------------Iv's-----------------Ability--Nature---Ball--Byte Filler

The offset at the beginning of the tables is the 148D980.

First of all, for the opponent's Pokémon to have EV's / IV's custom it is required that the opponent's Pokémon, any and all
one has a custom moveset, and of course it is also required that each Pokémon holds an item, it can be any one.

How do I apply this to a coach? Simple. Do you see the Index number of each table over there?
Now just go to the trainer editor, choose the trainer, customize your Pokémon's moveset, and equip some item
in this Pokémon. Now in the "EVs" option, in the box next to you you will put the number of one of the indexes up there.

If you want your first Pokémon to use table 1, in the "EVs" option, put 0 because 0 is the index of the 1st table.
If you want your Pokémon to use table 2, in EVs just change the 0 to 1. Simple. 

Each Pokémon on your team can use a different table, for example your 1st Pokémon can use table 1 and the
your 2nd Pokémon can use table 2, your 3rd Pokémon can use table 20, etc.
Just follow the configuration rule.

Altogether, as I mentioned earlier, there are 255 different tables, which gives you a huge range of configurations, this will provide an extra attribute
to make even more difficult battles against the coaches, in addition to giving an "air" of a more real challenge to the rom.