I would like to pulverize something that is not in my mod, nor in vanilla.

It is an entity that exists within someone elses mod.

 

So how do I do this anyways?

Is this possible?

 

if (result.entityHit != null)
        {
            int i = 0;
            if (result.entityHit instanceof EntityLivingBase)
            {
                i = 4;
            }
            result.entityHit.attackEntityFrom(DamageSource.causeThrownDamage(this, this.getThrower()), (float)i);
            worldObj.playSound((EntityPlayer)null, posX, posY, posZ, SoundEvents.BLOCK_GRASS_BREAK, SoundCategory.NEUTRAL, 0.8F, 1.5F / (worldObj.rand.nextFloat() * 0.4F + 0.8F));
        }

1. Add the mod jar to your project build path. Since 1.8.9 Forge automatically deobfuscates mods in the dev workspace, so you don't need a special dev version of the mod. You'll also need to tell gradle where the jar is, the easiest way of doing this is to place the jar in <mod folder>/libs(NOT <mod folder>/build/libs).

2. Use an instanceof check as normal, but first check that the mod is loaded with Loader#isModLoaded(modid).

Alternatively, to not rely on the other mod at all: If you know the EntityName, you can simply check if the mod is loaded, like Leviathan states, and then use the EntityList#NAME_TO_CLASS to get access to its class directly.