The following may be out of date.
Meditation reduces not only your personal damage, but also that of your summons. So, ideally, only use it when you're not in combat. (Note that the damage output of a summon is determined when it first appears, and does not change if you turn Meditation on/off afterwards. So you can turn on Meditation after creating your summons and they will continue to fight at full damage; on the other hand, if you summon something while Meditation is still on, turning it off won't fix the problem for that summon.)
Use Turtle to "tank" a monster (keeping it tied up in melee) while your Flamespitter blasts it.
You can have one active summon at a time for every 10 points of Cunning. So, you want to get your Cunning up to 20 as quickly as possible, letting you have 2 monsters at once. Later, aim for 30 Cunning to get a third monster -- but that can wait a while.
Hydras are very powerful, but they won't use their breath attacks if you would be hit. So for best results, try to stay behind the hydra. (Hydras have been observed to occasionally use their breath attacks when their summoner is at the edge of the AoE range. This can be lethal for the summoner, so again, try to stay behind the hydra).
You don't get any monster detection capability, but you can fake it by using Summon Control on an expendable summoned monster (and, face it, they're all expendable). Take control of something and use it to scout ahead.
Get your healing talent (Nature's Touch) quickly. Not only can it save your life, but it can also keep escortees alive.
War hounds gain physical power based on talent level, and deal more damage than stone golems at level 5.
You can prevent your main character from acting poorly when you control another monster by customizing the AI and leashing it to an anchor (with radius 1).
By midgame, you'll want to use mostly drakes, as they are at once great ranged nukes, good tanks and OK melee combatants. Set their skill priorities so that they roar early (it deals physical damage, confuses enemies and never damages allies), fire breath later (deals a bit more damage, but can hit you) and devouring flame never. With Frantic summoning and Nature's balance properly used, you can have 4 drakes in 5 turns.
Grand arrival makes hounds and ritches very useful in late game. Their arrival effect can drop enemy resistances to negatives, letting drakes wreck them. Drake arrival drakeling can be used as a shield, but is a nuisance most of the time, as the it takes a summon slot and can't be recycled.