"One or two attacks"
That just tells me you've either (a) never played the game at all, or (b) never faced anything higher than CR2 where creatures have multiattack.
Not everything is a blob monster with no brain. There is a chance targets change. Not always, but sometimes.
But let's just say your multi-attack does make a difference from my purely hypothetical, off-the-cusp statement of blocking "one or two attacks." Let's say there were three creatures, each with two attacks, and they used them all on the wizard (for some reason, even when three 6'6 barbarians are trying to cleave them in half and seems impervious to their blows! Let's just say these three non-thinking attackers (we'll say they are CR1 hippogriffins) all used their six attacks on this wizard. Three of those is a pretty easy fight for a second level wizard and his group of second level barbarians, especially with a shield spell that breaks combat mechanics.
So they all attack this wizard with a shield spell that breaks the game's mechanics. +5 to hit against our wizard's 17 AC. Six attacks at +5 going against a 17 AC. Heck, we'll give him a great dex too and make it 18. Six attacks against an 18 AC. There is still a 35% chance each one hits. That is close to a 90% chance this wizard gets hit once for 8-10 damage depending on whether it is beak or claws. There is close to a 50% chance the wizard gets hit twice, which will exceed his hit points.
That is a basic example of where I fail to see this spell breaking the game. There are so many examples I can think of that make this spell good, and useful, and fun, and strong... but not overpowered. Not by any stretch of the imagination. Which was my point - I simply asked for
examples of how and when this breaks combat mechanics.
PS - To suggest I am a noob (I'm not. I have played since the Blue Box came out.) might be construed as insulting. But even if I were, why not explain it to the person asking for examples, rather than making them feel bad about not being as "experienced" as you? Wouldn't that be the better of option?