I suggest avoiding this. It's light and sometimes amusing, but mostly empty. It's a "cute girls do cute things" show, but in a fantasy setting with heroes, elves, dragons, demon lords, etc.
Episodes 1, 5-6 and 11-12 are okay. I won't actually call them good, but at least the episodes have a narrative direction. The rest is anime marshmallow, the kind of nothing where you realise halfway through that you were asleep with your eyes open. I quite like some of the supporting cast, but the four main heroes are a blob.
1. Yuusha is an enthusiastic idiot whose goal is to become a hero (even when she is one). She's a PreCure Pink Leader, basically.
2. Fai Fai is an enthusiastic idiot who likes to eat and can run around jungles, talking to beasts. She's basically a wild animal with boobs. When hungry, she'll become non-sentient and chew on Seira's ear.
3. Mei is a one-dimensional otaku who collects the magic cards that let you cast spells in this universe. I found her a bit tiresome, actually.
4. Seira is a serious-minded elf who's bad at tidying up.
Theoretically, that's a workable core cast. Fai Fai can be entertaining, while Seira achieves the rare feat (in this series) of having more than one level to her characterisation. In practice, though, putting them into a scenario makes it less interesting. They flatten scenes. They don't have a direction of narrative travel. They just bounce around and sound cute. Trying to describe their effect on stories made me want to use the words "bimbo" and "airhead" as verbs. Look at Yuusha at the start of ep.7, for instance.
This is the kind of show where (in ep.4) you can meet a race of mackerel people who say the Japanese for "mackerel" at the end of all their sentences. (That alone, saba, is enough reason, saba, to delete this series, saba.) They're man-sized talking fish with human legs. The male ones are blue, while the female ones are pink. Oh, and when something burps, they don't burp. They say the Japanese word for "burp".
I liked some of the characters, though. The best is Mao-chan. She's the Demon Lord who gets defeated at the start of ep.1, only to get sent back in time to become the girls' schoolteacher. (Yuusha bungled a magic spell.) Mao is a tiny girl with devil wings and horns. She starts out trying to sabotage the girls' future, but before long gives up on that and decides that she prefers this schoolteaching thing. There's also a princess who's always wanted to marry the hero and isn't deterred by small matters like gender. The show soon loses interest in the princess's lesbianism, though, as it does with the hints in her debut episode that she's seen through Mao-chan's facade. (This is hastily explained away in ep.12.)
Eps.11-12 are functional light drama, i.e. they're head and shoulders above the rest of the series. I was amused by Mao and Seira's brief slob alliance, for instance. Unfortunately, the plot developments aren't always logical. Mao-chan's actions at the end of ep.11 make so little sense that even she admits this after the end credits. "I got caught up in the moment and did it without thinking." There's also a predictably over-convenient happy ending.
It's likeable, as you'd expect from a "cute girls do cute things" anime. The girls are fluffy, dim and nice. Its episodes pass painlessly, but you might realise afterwards that you weren't fully conscious for all the running time.