Short Answer: Multiplying base damage has the same effect as multiplying attack.
Explanation:
Confirming the damage formula with the manual:
Damage is calculated by multiplying the attacker's Damage statistic by its Melee or Ranged Attack statistic (depending upon the form of the attack) and dividing by the defender's Melee or Ranged Defence statistic
So we have
Total_Damage = (Base_Damage * Attack) / Defence
If we multiply Attack by 2, it is going to be equivalent to multiplying the Base Damage by 2, because multiplication is associative
(Base_Damage * (2*Attack) ) / Defence = (2*Base_Damage *Attack) / Defence
Example: Attack=20, Base_Damage =30, Defence=10
(30 * 20) / 10 = (30 * 20) / 10 = 60
Doubling Attack
(30 * (2*20) ) / 10 = (30 * 40) / 10 = 120
Doubling Base_Damage
( (2*30) * 20) / 10 = (60 * 20) / 10 = 120
Generally, a summon is there to be a meat shield. You want it to take retalitations and attacks, while your real creatures stay safe and then strike themselves.
To do that, you want high speed, good defense melee fighters. They go in before your real troops, take the brunt of the attack and then your creatures mop up the rest.
What you want to avoid are casters, as illusions don't have any mana (that's a weakness compared to nature; summoned nightmares, water elementals or faerie dragons do have mana available)
In cases, where the enemy army consists of fairly slow melee creatures, you may want to clone a ranged creature to get more firepower.
But otherwise these creatures are your best bets:
- Griffin - fast, durable and unlimited retaliation
- Ghost - deceptively durable due to their defense, their low hp will summon you a lot of them
- Air elemental - see above
- Vampire - can sustain himself, also fairly low hp, so higher numbers. For vampires to work, you need to summon enough to be a threat, though.
- Minotaur - their block special is very good and can help them survive several attacks even if their number is low
- Efreet - extra damage due to the flame shield
- white tiger and nomad - very strong lvl 2 creatures
I'm not a fan of summoning lvl 4 creatures with illusions, that's another weakness compared to nature. You get 0.2 griffins per level, yes, but you also get 0.143 phoenixes, That amounts to 39 hp summoned per level and outclasses phantom image pretty much immediately. With hit point based summons, I think you are better off summoning larger numbers of weaker creatures.
Also keep in mind that summoning spells on heroes usually only make sense in campaigns. On a single map the hero will have trouble keeping up with army growth to summon an amount of troops significant enough to have an impact and espeically with order your are then better off using other control spells like blind or berserk.
Best Answer
There is no golden rule, as too many factors can affect battle result.
But I can give a few hints on how one can understand battle difficulty:
You should know number of creatures you are going to face. Without scouting skill / mods you can see only estimate size, which translates as (source):
All creatures have level - from 1 to 4. It's rough estimate of creature power, let's say that creature of level 2 will always beat any level 1 creature without hero's bonuses in 1 on 1 fight. You should learn creature levels or use some cheatsheets for it - for example, Nature Creatures. It's not a good idea to attack Dozens of 4 level creatures without at least Band of your own level 4s.
Learn creatures special abilities. There are lots of them and they are hard to remember, but you can always check them during battle. For example, Efreet has Fire Shield, which reflects some % of melee damage back to attacker, it's better to shoot Efreets from distance!
Difficulty affects enemy tactics. For example, on next-after-normal difficulty enemy creatures begin to wait instead of rushing onto your army. Also with higher difficulty neutral creature packs become stronger, and sometimes strong packs prevent AI players from expanding.
Fights against armies with heroes are always more dangerous as heroes provide bonuses to creature stats. Heroes itself can cause a lot of problems with smart use of artifacts / spells / potions.
Armies inside towns have advantage because of walls and towers. So plan your sieges carefully.