The Elemental Wheels:

Stronger element -> Weaker Element

Fire -> Ice -> Wind -> Earth -> Lightning -> Water -> Fire

Light <-> Darkness

Psychic -> Spiritual -> Fighting -> Psychic

So, Fire is strong to ice, which is strong to earth, and so on.

Light and Darkness are directly opposed, so darkness attacks are strong 
vs. light creatures, and light attacks are strong vs. darkness 
creatures.

Monsters have one level of optimized armor vs. attacks of the element 
they are strong to and their element.  They have level 1 weakness (take 
double damage) from the element(s) they are weak to.  If there is a 
Water and Fire monster, however, beyond a good explanation 
mechanically the creature's strength and weakness to water would 
balance out, so that the creature is strong against Ice and Fire, but 
weak to Lightning.

All monsters must have at least one element and can take up to three.  
A monster can learn and use attacks from its element more easily.  
Mechanically this makes the attack deal an extra ten base damage.

Neutral, or untyped attacks, are learned normally.

Attacks that are not part of its element are harder to use, and have a 
10 damage penalty for the monster to use.

Attacks that the creature is opposed to (Ice and Water for Fire) have a 
20 damage penalty for the monster to use.

Combination Techniques:

  Monsters usually have their basic abilities, but sometimes trainers 
can help a monster through training, encouragement, fear, or whatever 
other sappy or creepy reason you want to come up with.  Fact is, 
trainers can help monsters fight and do things better.  To do this, the 
trainer and the monster each must have trained together somewhat and 
have spent CP in the ability they wish the monster to attain.  The 
cost for the monster is the base technique: The ability the monster can 
do normally without the aid of the trainer.  The cost for the trainer 
is 1/2 that of what it'd cost him to take normally.  He cannot use this 
ability, but he can train a monster to do it under his direction.

  The cost for using this is CP/round in EP to the trainer, using the 
normal CP cost of the ability, if in combat or strenuous situation.  If 
not in combat or other very strenuous situation, it is extended to 
CP/minute.

  Activating the combo technique takes an action for the trainer and an 
action for the monster.  Usually a monster has another technique tied 
to the activation of such an ability:  The monster can have one (and 
only one) ability (or combination of abilities, see below) tied to the 
activation of the trainer's techniques if they wish.  If the monster 
does, the monster must always activate this ability when the trainer 
does its tied combination ability.  If the monster does some other 
action, cannot activate the ability, or otherwise cannot do so, the 
action is lost for the trainer.

  Multiple abilities can be tied to one activation for the trainer or 
the monster.  However, if this is so, they cannot be activated 
separately.  It's all-or-nothing.

Items:

Using an item is a combat action, so you can only use one item per 
round unless you have extra attacks.  Potions and ethers can only be 
used on monsters.

Minor items:
  Potion - restores 30 hit points
  MID - Monster Identification Index.  This basic hand-held booklet is 
full of information on identifying monsters and monster types for 
unknown monsters.
  Ether - restores 20 energy points

2 Minor items:
  Hi Potion - restores 60 hit points
  Hi Ether - restores 40 energy points
  Remedy - This basic cure-all removes all status effects (drained 
stats, burns, poisons, irritants, tangle effects, blindness, etc.) from 
the monster.

3 Minor items:
  Hyper Potion - restores 90 hit points
  Hyper Ether - restores 60 energy points

Major items:
  Ultra Potion - restores 120 hit points
  Ultra Ether - restores 80 energy points
  EMI - Electronic Monster Index - This device (dubbed by many the 
Pokédex for some unknown reason) is a speaking encyclopedia on monster 
habits, types, and abilities.  It has limited scanning capabilities to 
discern and identify more common monsters.  New information is uploaded 
on a weekly basis.  It is most often given to new trainers by their 
village monster expert when they take the path of a monster trainer.

2 Major items:
  Max Potion - restores all hit points
  Max Ether - restores all energy points
  Rare Candy - Instantly grants the monster this is given to 5 
temporary CP in attributes it already has for one event.  Cannot 
increase skill points.  Rare candy does not stack with itself.

3 Major items:
  Elixir - restores all hit points and energy points.

5 Major items:
  Elemental Stone - Causes a monster to evolve into a new monster, 
possibly changing its type depending on the stone used.  A monster can 
only evolve once, and almost always with a stone that is of its element 
or of an element not on its 'wheel'.  This boosts the monster's CP by 
10 permanently, investable in new attacks and abilities or old.  The 
monster cannot increase skill points with this.  The monster and 
trainer(s) for the monster lose all combination techniques that they 
had beforehand.  They can still re-acquire them, but it takes time...

   Stone names:
     Charmandstone, Sandshrewstone, Pidgystone, Laprastone, Pikastone,
       Squirtlestone
     Espistone, Umbarstone
     Hypnostone, Ghastlystone, Hitmonstone
   I'm sure you can see the horrible ripping I'm doing here, but hey, 
it's late.  I think it's funny.