My TH became level 7, and I want to use new troops, such as Dragon, Minion and Hog Rider. What's attack strategies you recommend?

full dragon is the best at th 7

Troops that are unlocked in town hall 7 like the dragon, as well as the minions and hog riders in the dark barracks can be a little overwhelming at first especially if it is your first time to use them.
In using dragons, you have to keep in mind the placement of two kinds of defensive structures.
The air defenses and the air sweeper.
These two defensive building are your dragons greatest enemy.
They can make or break your attack so make sure you plan your attack beforehand.
Do not deploy dragons on the side facing your air sweeper so that it cannot blow your dragons back.
This will give your dragons a hard time in reaching the target and other defensive structures, like the archer tower and the wizard tower, can attack them repeatedly causing extreme damage.
Destroy one air defense using the lightning spell first then deploy your dragons from the side from which your troops can reach the remaining air defense the fastest.
Dragons in town hall 7 can be upgraded up to level 2 in the laboratory.
Level two dragons are much stronger than the level 1 dragon so it is important that you upgrade it to make your attacks more effective.
If you wanted to try troops from the dark barracks, you can use the minions in combination with other ground or air troops.
Deploy your minions on where the defensive buildings that can only attack ground troops are placed.
Example of ground troops are cannons and mortars.
Minions are air troops so the ground troops cannot attack them.
This way, your minions can kill them single mindedly.
Hog rider on the other hand can be used not just in attacking enemy for loot but also in clan wars.
Their speed is their strongest feature, making defensive buildings find them hard to catch and destroy them immediately.
Drop your hog riders on the side where the most defensive building are.
Hog riders attack defensive buildings.
Use them in combination with archers.
The hog riders will take down the defensive structures and your archers will take care of the other buildings.
It is best used with the heal spell to keep them alive while inside the camp.
Drop the heal spell where the hog riders are concentrated during attacks.
The downfall of hog riders is the big bomb.
Big bombs can kill a group of hog riders in one explosion.