Knot the thread, but do not cut it
As a very general rule for Limited, I would look to play something like this: 1 mana: 02 creatures 2 mana: 46 creatures 3 mana: 35 creatures 4 mana: 4 creatures 5 mana: 3 creatures 6+ mana: 12 creatures That's far from hard and fast, but it's a good place to start
The thing that makes this card slightly more annoying and therefore slightly less played is not that you don't kill the thing you bounce but that so many cube cards have enter the battlefield effects you don't really want to give them again
While its really just a bunch of keywords thrown onto a vanilla body, Sire of Seven Deaths is still one of the best new cards in MTG Foundations
In the simplest case, you'll only have basic lands (Plains, Island, Swamp, Mountain, and Forest) to work with
If the spell you cast has in its mana cost, you must choose 0 as the value of X