RAW, it says that Concentration may be broken when you take damage. This will interrupt sustained Concentration spells or readied spells. Concentration is also required to cast spells with longer casting time than a single action.
While concentration may not be needed for quicker spells, "mental focus" is.
Casting in Armor
Because of the mental focus and precise gestures required for spellcasting, you must be proficient with the armor you are wearing to cast a spell. You are otherwise too distracted and physically hampered by your armor for spellcasting.
I think it's reasonable to say that getting injured would be more distracting than wearing unfamiliar armor!
While not explicitly stated by RAW, a trigger of "I attack the Wizard when he starts casting a spell" should work.
You attack "right after" the Wizard starts casting.
The example triggers are split-second things that interrupt movement.
Examples include “If the cultist steps on the trapdoor, I’ll pull the lever that opens it,” and “If the goblin steps next to me, I move away.
"If the cultist steps on the trapdoor..." - you're not waiting for him to step there & stand around for the rest of his round, if he tries to walk over the trapdoor, you're pulling that lever the moment he steps on it, interrupting his move action.
Same should apply for spellcasting, unless maybe they have a "reaction" casting time (taking a fraction of a second, too fast to interrupt).