I am level 67 and trying to get the kill a legendary dragon achievement. The skills I currently need help powerleveling are:

All the crafting skills

Speechcraft

Destruction

Conjuration

Can anyone help me level these up?

Some of these aren't too easy to powerlevel but here are some ideas that might work. Sorry if some aren't helpful, I don't usually level up fast so I'm not sure if some work.

This one works for me since I wanted to raise speechcraft faster. Works only if you don't have much problem with having a bounty on your head. Get a very low bounty in a hold, head to the hold's major town and speak with a guard. He'll say "wait, I know you", you'll say "you're making a mistake" and after that you get several options to answer with. Choose the persuasive one convincing him you're not worth it and if it works out it'll raise speechcraft a nice portion without if you have the Thief Stone active. It also doesn't remove the crime level so you can do this to every guard you can find in the hold. According to uesp you'll always have this option after lv 75 with speechcraft, and dialogues will reset in 2 days when you'll be able to visit all the guards again.

If you wish to train in Speech Ogmund in Markarth (specifically at the Silver Blood Inn) is an expert trainer which means he can get you exactly to lv 75.

Crafting skills are easy, for alchemy get some potent ingredients and mix them. For example, mixing blue mountain flower and blue butterfly wings are very common ingredients and will increase the skill pretty nicely with the Thief Standing Stone. Blue and orange dartwings, or garlic and slaughterfish eggs are another good combinations that feature common ingredients. In general, the more expensive the potion you make the more experience you get.

For enchanting is the same, more price = more exp. So if you haven't disenchanted for every effect yet you'll get the most out of Daedric armor pieces. Failing that, there's no other trick with this, get a lot of soul gems by either buying filled ones, filling them yourself or both, and enchant craploads of stuff. Remember to change from The Mage stone to The Thief one before selling everything you've enchanted.

Smithing one is easy, no special trick. Just buy as much leather and ingots as you can and smith stuff, like hide helmets or iron daggers. And all while having The Warrior stone.

Destruction is damage based, so aside from training the only option is to get out there and use the most damaging destruction spell you have. No special trick to it, aside from enchanting your gear to make destruction spells free to be able to spam much more spells. You could also try potions that fortify damage from destruction spells.

Conjuration can be combined. You can get the conjuring spell that lasts "forever", like a Thrall, which will constantly raise Conjuration a little bit while you're in combat, thought that one is hard to get, you can summon normal creatures too but will have to recast them over and over. At the same time, when fighting use only Conjured weapons. Making damage with conjured weapons will raise Conjuration along with the weapon skill governing the weapon you're using, so if that's not maxed out it'll help some too.

If you don't mind using bugs, another way to increase conjuration really fast is to constantly cast Soul Trap on a corpse.