[@UnusualStranger][@WriterRaven] Let me clarify.
First of all, I'm pretty sure the shock wave should at least hurt and daze the Nightshade. I mean, he's still a human, and he doesn't seem to wear too much armor. Secondly, the grenade will not actually work like a frag grenade; its casing will remain more or less intact.
Thirdly, I'm no longer sure what exactly hurts a shadow. I now guess that they are not dazed by light, so would throwing it to my legs -bad idea; I'm not invulmerable to it- hurt it from the flash, or the explosion itself? And as of the distance between the grenade and me, don't forget that Jonathan has just stepped into the bar, and therefore he is still pretty close to the door.
Now, as for the construction of the grenade, it is not nearly as complex and sophisticated as a modern, real-life concussion grenade. The secret is mostly to the shape of the sphere; the six holes are very thin, just to contain the explosive material, and to prevent part of its corrosion (even though it is not perfect;as is the case with all my explosives, they require some periodic maintenance), while the rest of the case is thick enough to contain the explosion. Even though the minerals used are obviously not good ol' TNT, they are not picked with the same care they are for the US army; a mix of potassium and phosphorus is probably all he needs as basic materials, which I can guess he can acquire in small quantities in some places.
Furthermore, the flash grenade (though definitely handy) was not an asspull, even though I can understand what lead you to think of that. I do have a certain canon assortment of Jonathan's gadgets in my mind, which I guess I bought to give you now in some detail:

2 Flash Grenades (of the kind used above), loaded on his belt
2 Smoke Grenades (simolarly constructed, those use a slow-burning black powder and sulfur compound to produce Smoke for 10-13 seconds), loaded on his necklase
1 Ballistic Knife (spring-loaded; comes preloaded with 1 blade, but more can be made on-field from avalaible materials), loaded on his belt
1 modified SCAR (Storm Core Abnormality Detector) Sensor (can detect Elemental activity in the near area; far from perfect, it requires either a constant use of Elemental abilities or a giant showcase of Elemental power to have happened at an area; on the other hand, if the requirements are met, the sensor can recognise Elemental activity up to 15 minutes after it happened), mounted on his necklase
1 Chainmail ... Well, chain ( mostly used as a whip; because of the metals used in its work, Mettalics like Jonathan can manipulate the chain mid-swing to target things better), mounted on his necklase. Be could probably use it to try to hit the Nightshade as he passed past him, but he already had the flash grenade on his hand.
1 Detection Chip (when it senses it has been planted on something or someone ,it sends pulses of energy similar to Elemental activity, which after a couple minutes can be recorded from the SCAR. It has a range of up to 25 meters. It works best when planted on Elementals or other magic-imbued objects), fit into is pouch
1 Pouch of spares, filled with complex parts that be cannot make without much time avalaible, or specific tools), mounted on his belt
Feel free to ask me to edit or remove anything that could break the balance of atmosphere of the game.

And finally, not flinching might have been a poor choice of words. Think less of invincibility and more of steely (forgive me for this pun) determination; like when a normal person gets punched, but keeps his ground and quickly moved his head back into position.

Again, Im so terribly sorry about so ignorantly breaking the games flow. Maybe Im not as suited for the whole thing after all... But God damn I'll try to become.