quick guide covered a lot of stuffs actually. you may also want to visit summoners war on reddit, they have a whole list for newcomers also

slot 1 main stat is fixed to ATK (flat value)
slot 3 main stat is fixed to DEF (flat value)
slot 5 main stat is fixed to HP (flat value)

slot 2 main stat can be ATK/DEF/HP (both flat and %), or special stat SPD
slot 4 main stat can be ATK/DEF/HP (both flat and %), or special stat crit rate% / crit dmg%
slot 6 main stat can be ATK/DEF/HP (both flat and %), or special stat accuracy% / resistance%

ATK/DEF/HP% scale much better when you are high level compared to the flat values (example, a 6* ATK% rune at +15 is 63% of your base stat. with all 3 slots 2/4/6 set to ATK%, you can get 189% boost to attack stat!). this is how you can see attacker with 800+2000 attack or tank with 12000+30000 HP

Also, the special stat is needed to build specialized skillset, such as Bernard that always has the first turn (with swift runes 25% speed and 6* slot2 SPD rune adding 42 speed and speed substat on every runes), 100% resistance Rina, which will almost never get stun, etc.

wind inferno is fusion material to get better monster, so it's still a good summon.

light fairy is quite decent. if you could do faimon map 1, i'd suggest you to grind it to level up you monsters, get fire hell hound to max skill your sieq and hope for fire inugami to drop - that's the single most important monster you need that can help carry you to mid-late game. Try to collect the magic & element essence to awaken your monsters too - water fairy, light fairy queen, fire hellhound, wind inferno.

For crystal, i'd suggest you to collect until 750 to purchase the premium pack. it is much more cost-effective. Don't worry coz this game actually gives quite a few crystals - just look out for new events in the news section. For newbie, you should be able to summon the premium pack within a month or so.

later in game when you have a decent line-up, it will be more beneficial to use the crystal for experience boost & energy refill to push up the monsters to 6*

I do agree with wei xuan, is not difficult to keep 750 crystals to buy premium pack.

For me, I would start focus on hellbound as the double attack and begin hunt can help you a lot when the game start getting difficult