TACTICAL S
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S AHEAD...
OK, in general, you seem to be doing well....
The idea of "best tactics" is very personal.
Accord already mentioned that you are usiong a LOT of tanks.

I do that when I use a party. I prefer
STEEL to SPELL.

Others prefer a better balanced attack, still others are more mage-based...
Sun Tzsu taught "To win without fighting is best." His whole idea was to use self-knowlege to gain the upper hand. Know your strengths. Know your weaknesses. Use one, avoid the other.
Your main Weakness:
THE WHOLE WORLD IS OUT TO GET YOU !!!! Fear that. Learn to use one of your rangers, or thieves, to scout ahead of the battle.... Cause as much damage to the enemy while keeping yourself out of range. For your party, I would recommend several ideas.
1) Get some bows for your fighters. Ranged weapons will hurt the enemy, weakening them BEFORE they reach you.
2)Use the summoned creatures as a buffer zone. Get them in before you attack the enemy. Keep them between you and them, an use your ranged attacks on them while they are occupied with the summoned ones...
3)CONCENTRATE your attacks. Wounded enemies will hurt you as bad as full-strength ones. Concentrate your attacks on one enemy, then attack the next when it dies. The exceptions are magic users. Ranged weapons will disrupt their spells, so use that. Attack the spell-casters, then concetrate on one. When anyone starts another spell, shoot him again...
4)Use the battlefield to your advantage. This is why scouting is so important. Use corridors, doorways, bulding, WHATEVER to try to keep them comin one-at-a-time. Send in a scout, have him fire at one enemy when only that one is exposed. Run back to the protected party. Concentrate fire on the enemy, one-at-a-time...
Mix and match these ideas, rinse and repeat....

Others will put in their $0.02, and they will also have good points. These worked for me...
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The RudeDawg
Known in these Forgotten Realms as Perin LightEyes
and my girlfriends, Pamila and Pfil