Worlds of Magic: Planar Conquest (UP)

This trophy does require some luck in obtaining a Water Elemental. They can be recruited from world features, such as an Inn or rescued from a prison, and can even show up through positive events. However the odds of actually getting a Water Elemental are very slim. Thus, my recommendation is to use these steps:

1. Create a custom Sorcerer Lord.
2. Put 9 points into the Water Magic Circle. Select whichever spells you want.
3. Put 1 point into the Protection Magic Circle. Learn the Bless spell.
4. Start the new game.
5. Put all Power into Research points.
6. Now is where the luck part comes in. You need to successfully learn the Water Elemental spell from the randomized spell list. By putting max points into the Water Circle and only 1 point into the Protection Circle you will have the best odds of that Tier IV Water Elemental spell showing up.
   
   You can keep restarting games to change the initial random 8 research spells but that gets tedious. You can also try the Wizard's Roulette spell (you always start with it) which allows you to reroll the spells that are available to research.
    
7. Don't even leave your starting capital. Focus on making it stronger and defensible while you dump all possible points into researching new spells. You should be able to blast through several turns at a time.
8. Once you get the Water Elemental spell you now have another obstacle: you must get your Spellcraft to 34. Bless costs 12 Mana and Water Elemental costs 22 Mana.
9. When you are ready you can finally take your army and attack anything you can find.
10. Cast Water Elemental on your first battle turn then Bless on your second turn. The trophy will immediately unlock at this point.