Magic and class levels can have an unpredictable effect when combined with non-player creatures. You may be better off adding specific spells and features, rather than class levels. Since Storm Giant already have innate Spellcasting, that makes adding spells much easier.
However, beyond that, your better option might be introducing Legendary, Lair, and Mythic traits:
Legendary actions shift action economy in the monster's favor and significantly sway the tides of battle.
Lair actions provide unconventional options, such as terrain manipulation and secondary damage/buffs/debuffs. (A Lair action could include something like summoning X Air Elemental.)
Mythic traits create an opportunity for phased combat, so that even if the players manage to defeat the Storm Giant quickly, the combat will escalate and continue at a higher difficulty.
Wizard levels are tough because WOTC has openly admitted that you can take a given set of spells for a monster and swap the spells out for spells of the same level and functionally radically alter the CR, but the CR rules don't care about things like save-or-sucks you might give the giant. All it pays attention to is damage done. That makes it even harder for you to sanely balance things.
Volo's has a CR 16 Storm Giant, if you want: a Storm Giant Quintessent. It's the second highest CR giant in the game, behind Borborygmos. Note that the Quintessent is much, much less dangerous if you can get it outside its lair, but its lair can be any thunderstorm, so you can have it carry its lair with it if you want.
The usual tricks apply for making it a tougher opponent without raising its CR: giving it proficiency in combat-critical skills like Perception and Athletics won't raise its CR. If you want the CR higher, thematically appropriate minions - like a Storm Giant servant or two - are probably best. It's very difficult to scale up a single monster and have it be a credible threat - the Tarrasque is incredibly easy to defeat, for example.
A quintessent and a regular storm giant would be a "hard" encounter for 6 level 14s which means it would be quite easy for them unless they have already used a fair amount of their resources, adding a second strom giant would turn it into a challenging encounter if they have all their daily resources.
Magic and class levels can have an unpredictable effect when combined with non-player creatures. You may be better off adding specific spells and features, rather than class levels. Since Storm Giant already have innate Spellcasting, that makes adding spells much easier.
However, beyond that, your better option might be introducing Legendary, Lair, and Mythic traits:
Legendary actions shift action economy in the monster's favor and significantly sway the tides of battle.
Lair actions provide unconventional options, such as terrain manipulation and secondary damage/buffs/debuffs. (A Lair action could include something like summoning X Air Elemental.)
Mythic traits create an opportunity for phased combat, so that even if the players manage to defeat the Storm Giant quickly, the combat will escalate and continue at a higher difficulty.
Wizard levels are tough because WOTC has openly admitted that you can take a given set of spells for a monster and swap the spells out for spells of the same level and functionally radically alter the CR, but the CR rules don't care about things like save-or-sucks you might give the giant. All it pays attention to is damage done. That makes it even harder for you to sanely balance things.
Volo's has a CR 16 Storm Giant, if you want: a Storm Giant Quintessent. It's the second highest CR giant in the game, behind Borborygmos. Note that the Quintessent is much, much less dangerous if you can get it outside its lair, but its lair can be any thunderstorm, so you can have it carry its lair with it if you want.
The usual tricks apply for making it a tougher opponent without raising its CR: giving it proficiency in combat-critical skills like Perception and Athletics won't raise its CR. If you want the CR higher, thematically appropriate minions - like a Storm Giant servant or two - are probably best. It's very difficult to scale up a single monster and have it be a credible threat - the Tarrasque is incredibly easy to defeat, for example.
Against 6 level 14 PCs, it takes a CR 21 monster to even qualify as a medium encounter. Two giants at once would be a decent challenge.
The other option is to give to add a minion or 2.
A quintessent and a regular storm giant would be a "hard" encounter for 6 level 14s which means it would be quite easy for them unless they have already used a fair amount of their resources, adding a second strom giant would turn it into a challenging encounter if they have all their daily resources.