These are the two achievements that you need to spend time grinding in order to complete them. Magic Tactics and Weapon Tactics require you to level up one magic spell and one weapon type to level 16 respectively. Because you won't be able to reach level 16 in a normal game play, I've listed the strategy below on how to easily get them.

The easiest spell to reach level 16 is "Cure", as you can use it outside of the battles and gain experience. The trick is you want to damage yourself, then using "Cure" to heal everyone.

Strategy:

1. Go to Mysidian Tower, 3F, you'll find larva floor there.
2. Step on the larva tile, you will take 1 damage.
3. Open your inventory -> magic, then cure everyone individually.
4. Repeat step 1 until you reach level 16.

The problem with skill level is that when it gets higher, you'll start the battle with a negative experience. It requires you to attack a dozen of times to actually gain a positive experience. Sadly, most of the monsters cannot withstand many hits.

So the trick is that instead of attack monsters, you attack your ally or yourself. Then you heal and repeat. Fortunately, there is a Blood Sword that can heal when attacking and this will come in handy for our strategy.

Strategy:

1. Open your Menu -> Configuration, change the "Cursor Memory" on. This will allow the game to remember your last cursor location.
2. Equip the Blood Sword on anyone and a strong shield on the other hand, then raise the Evasion to 99% so that the monster cannot hit you.
3. Kill all other allies to speed up the battle.
4. Then start a battle with an easy monster.
5. Attack yourself; with the Blood Sword you will take the damage then immediately heal yourself.
6. The next round, the cursor, will be targeted yourself, so you can simply hold down "X" button or "Enter" key (for keyboard) for several dozen times.
7. Repeat until you reach level 16.