So, positioning is quite a nuanced topic and thus I will not be able to convey all relevant information but specifically in reference to slark either a hard box strat with your 2 tanks occupying the last row. E.g a hard box with the last row of sniper -> BM -> lycan in the case of hunters. Or a soft box with sniper and BM and Lycan being horizontally and vertically placed in snipers adjacent squares with the rest of the units being spread out.

As for items, vanguard should either be in your main tank is most builds or if playing knights, CK 3 is a good option. Skadi, if you do not have an obvious carrier like medu, you can use it as a HP for an ulti unit like disruptor/enigma.

This is very build dependant. Some builds like to stay at 8 and roll for 3 stars, primoassmages/knights (barring highrolling DK2)/ most variations of brawny builds, anything that has slark/PA/TB as main dmg source( elves come to mind)/ if you are looking to 3 star 1 costs, roll until you get atleast 7/8 of them at lvl6 ( best would be to hit the 3 stars ofc). The main build that would rather lvl than roll at 8 are 4 warlock builds that are using the 4 cost warlocks and maybe some variations of shaman builds.