The converted mana cost for Shared Animosity is three. You will need one red mana and two colorless mana to bring Shared Animosity onto the battlefield. For what Shared Animosity can do, the casting cost is well worth it. Remember that this is an enchantment spell. It means that Shared Animosity is susceptible and vulnerable to spells and abilities that target enchantments.
What does Shared Animosity do in the first place?
If and when you attack with a creature you control, it will receive +1/+0 until the end of the turn for each other attacking creature that shares a creature type with it. In this respect, you want to be constantly aggressive with your creatures. But, you want to build around a specific type of creature for the most part. There are examples to be used in regards to Shared Animosity.
One example would be an elf deck. Say that you attack with five elves, all five of them would get +4/+0 until the end of the turn. However, things will get interesting if you attacked with seven creatures. Two of them are elves, three are elf warriors, and the other two are plain warriors.
The two elves would get +4/+0 until the end of the turn for the other elf and the three elf warriors. It gets pretty interesting when you factor in the sub-types together. When that is factored in, the power of those creatures get increased even further. This enchantment alone ensures that you can mess up your opponents with just a few creatures provided that they share the same type.
How do you get more out of Shared Animosity?
If you are playing with red cards, you can add in "Rage Reflection" which is a rare enchantment printed in "Shadowmoor." Rage Reflection gives your creatures Double Strike. Combine Shared Animosity and Rage Reflection together, you got a good aggressive creature deck going. Make sure that you build around specific creature types.
If you are playing a mono-red deck, you would want to build around goblins. Shared Animosity and "Goblin Assault" makes for a devastating combination. Goblin Assault produces a 1/1 red goblin soldier token at the start of your upkeep. If you have enough of those tokens on the battlefield, they beef each other up. To add to the injury, you can use Rage Reflection.
Overall, Shared Animosity is a good card. However, you have to build around a creature theme to get the most out of it. As long as you are playing aggressively, you will get a lot out of Shared Animosity.