And another one

Well, if you want to build a team around Jack, I would suggest probably Caster cause that 1/3 chance at Waver. Else I would go for rider because Ozy, Drake, Quetz. Otherwise just go for who you like.

I would say Berserker, because they're really good at those mixed class stages. You might not like them because they're squishy, but that's why you've got support servants to keep them alive with heals, and def buffs. Heck, you have Merlin, so there's nothing to fear. Unless you'll be picking Herc as your GSR ticket servant down the road (and getting him to bond 10), Berserker GSSR is the way to go.

Don't go for zerks if you don't like them. They are one of the most useful classes, but there's no point playing something you dislike. Also, only one out of the three berserkers in the GSSR is actually suitable for hard content, so it's a massive gamble.

In your case, rider would be the most beneficial since you seem weaker on those, which works fine since, as a star generator, Jack goes well with most riders too. Otherwise, caster is the best option.