When I finally leveled Alexander up enough to get shadow creature spells, I chose every shadow magic spell I could. The thought I had was a bit of Lord of the Rings, as in when Gandalf faced of against a balrog... a creature of shadow and flame. I thought the pairing of my pyromancer with shadow might be natural. Thus, I spent training points on all the shadow spells I could, which amounted to 4 additional spells. Not a very heavy investment.
I then played around a bit with the shadow spells and discovered that there isn't nearly so much flexibility for one wizard to play with. As a pyromancer, my heals come from damage spells, so Shadow Seraph is useless except for either self heals with fairy or spending more training for heal spells for others. Shadow Sentinel could be OK under the condition that Alexander was geared specifically to tank, but that's not normally a pyromancer's job.
That leaves 4 of the 6 shadow spells being useful for Alexander. Oh well, he still knows the other two. I think every shadow spell is useful - my theurgist definitely uses Shadow Seraph; but not every shadow spell is useful for every school of wizard. The big question was, might the shadow creatures find a bigger use in a deck than just being simply for curious variety. Could I build around shadow creatures?
After running some experiments, I quickly realized that this was a job best handled by sorcerors, not pyromancers. Pyromancers don't have any natural universal blades or traps. Creating a shadow creature build for Alexander would require item, pet and treasure cards in order to be at all effective. Also! More importantly, getting shadow pips. If you run a shadow creature build against anything that is in your ability range of damage, you will get crushed 9 times out of 10. You just won't get enough pips fast enough to kill anything. You will need shadow pip generation on your hat, robes, and shoes. And you will also want to get triangle jewels that boost shadow pip generation. The shiny pip hematites give +3% toward shadow pips; and they can be found (rarely) on various bosses in Mirage. Another helpful piece of equipment is Tamauji's Discerning Dagger (drops from Tamauji in Mooshu, but also available in the Bazaar). This athame has two triangle sockets, but otherwise negligible stats. This athame, along with any Triangle socketed deck offers your wizard the chance for +9% shadow pip generation from jewels. This is only available for wizards at level 115 or higher. If your wizard is at least level 105, you can equip the sockets with lustrous pip hematites (+2% shadow pip generation). With hat/robe/shoe maximums at 4/3/2, that means you can have a bonus of +18% shadow pip generation... which means nearly 4 out of every 10 turns you should be getting one. That's not too bad and might make a shadow creature build legitimately viable.
Another thing to consider is your raw damage source from gear. Universal damage isn't hard to come by, but gear mostly offers lower damage scores than school specific damage. That makes sense in terms of balance, but it means that your shadow boost isn't going to be great. Few pieces offer specific shadow damage boosts, and those boosts are all too low to consider the gear for that purpose. If you're getting gear from Kingsisle cards or booster packs, those tend to have favorable boosts toward universal damage. Also some bosses drop non-school gear that can be beneficial. Sands of Time gear can be helpful here, specifically the shoes and hat.
This is the crossroads though. Do you pick gear for the universal damage boost, or do you pick gear that offers universal damage blades and traps? Gear from packs and cards will get you both, so those are most favorable, but if you're not a big crown/cash spender, that's not going to be the answer.
Currently, with no trainable shadow spells, the only shadow blades and traps you're going to get come from the Malistaire robes and wands, and neither of those offers any shadow or universal boost to damage. The shadow trap is a nice choice because it won't get used up by other spells hit your target, nor be purged by sacrifices or dark pacts.
Since pets don't offer any shadow pip generation, you'll want to consider having one with talents that either offer defense (heals and resistance) or offense in the form of universal damage, universal critical, or spell cards like balance blades or hexes.
Well... that's where I'm at right now. I'm still farming for shiny pip hematites. So, I'm not using the gear I had chosen for my shadow creature deck. I will say that having experimented, I had from gear the following item card spells: 2 dragonblades, 1 balanceblade, 1 hex, 2 shadowblades, and 2 shadow traps. If I managed to get 1 of each of the boosts and cast shadow nova, I did pretty good damage - in the 20,000 range. This convinces me that a shadow creature build IS viable, but there's a number of things to consider - can you maintain a damage pace, and can you resist or heal enough to hang around before finishing off a boss.
In PvP, a shadow creature build would be hampered by the same things school specific builds are hampered by - high resist opponents (though less so, by the nature of shadow piercing) and getting tangled up with weaknesses.
In PvE, a shadow creature build would be VERY effective against bosses of all sorts, but would require enough of the right kind of blades and traps to dispatch bosses quickly. Dark shepherds might help to make a more solo friendly deck, but in group play, the number of spells other players could use to help you are limited. Sorcerers and necromancers would make great additions; otherwise, players would need to pack treasure cards to help.