Summon Suggestions

You could make summons limited to a certain number of uses, after that you need to capture a new creature. This would fix the problem if you capture a low-level creature that eventually become useless.

You could the creatures powers up with the player character, so if you find a sword, your minions get +1 attack, if you find a shield they get +2 hp, that should make them more durable in the long run.

Spell suggestions

I've also noticed that if you have two tabs with the same spell, the game tends to get pretty easy so I'd suggest instead of having two individual spell tabs which can be used separately, you could perhaps use them as one spell with is more powerful. To if you have to heals you can't use them as 8+8, but rather when you use heal you get +14 hp, powerful enough for it to be useful but not enough to essentially give you infinite life in many battles. Or you could remove one spell tab and upgrade each spell every time you find a new copy of that spell. For example the magic missile which does 12x1 damage is quickly overpowered by your normal attack, and it becomes worthless, however if every time you found a new magic missle spell, it did a little bit extra damage it would probably always be more powerful than the normal attack.

Power-up + multi-missle is pretty broken also, right now it does enough damage to kills pretty much any foe in the game other than bosses and some at level 5. I'd suggest reducing the damage by just a few points to that you don't one-shot everything all the time or limit it to 3 target at the most.

If the heal had an party effect for your creatures too it would be awesome.

Enemy Suggestions

It probably wouldn't hurt if you gave some enemies a mutli-target attack. Right now there are creatures with sticks and versions of those without sticks, those with sticks deal +1 damage, you could however do a double stick version which does two strikes and each strike doing the same as if they had one stick but divided over two targets. So if the stickless does 1x3 damage, the single stick does 1x4 and the double stick could do 2x2. These double stick version would first strike your pets of course. This could also be an extra power-up found in the dungeon.

It would be bit fun if there were kamikaze enemies which blows up and deals party damage at turn three of combat, such an enemy would make target prioritizing a bit more complex and interesting in some battles since you'd sometimes have to choose between killing the bomb or taking the blast and kill the other creatures.

Giving bosses minions and powerful heal spells at higher levels would make them a bit more challenging, right now they just trade punches with you.

If you want the game to go on for longer you could take the diablo approch which is to simply take an enemy and put a different color on him to indicate his power level. So there could be three different kinds of rats, a blue, green and red, each being successively stronger in some way.

Lastly, I'd recommend changing the battle system so that there is an enemy front line which is the only line which can use melee attacks while those behind can use weaker ranged attacks. This would eliminate the problem where you sometimes get bombared with three lines of enemies which get first strike. Once the front line is killed, you then leave the back-up open to your attacks and you're open to their melee attacks. Of course not all foes should have this range attack, perhaps only those which also have a stick option should be able to use ranged attacks. THis change would of course change a great deal in the balance of the game if you just add this system.

Hope you'll find something useful from this. Your game is awesome btw.