How to understand which creature is the best to be mimicked using Phantom Image or Create Illusion in Heroes IV?

Mages of Order in Heroes IV can actually prove to be good summoners, creating 44+4 per level HP of creatures with Create Illusion and 88+8 per level with Phantom Image, or 152 HP at 8th level (the minimum when level 4 spells become available). For comparison, Summon Griffins, level 4 Nature spell, creates 0.2*level Griffins. As Griffins have 95 HP, they effectively summon 19 HP per level, or the same 152 HP. Each level after that Nature summons 11 HP per level more than Order, but... there are also Genies that summon 18 HP per Genie, summoning being arguably their best utilization. Not bad, aye? So, not only is Order the direct counter to Nature with Banish, but it is also better at summoning than Nature, also having many more interesting mechanics to choose from. But that's not the point.

The downside of Nature summons is that you can only summon the creatures that you got in your Mage Guild, while the downside of Creat Illusion is that you need at least one creature of the respective type to be present on the battlefield for at least the first round(s), but you can summon litereally everything except for Black Dragons.

Not only do you usually not have a choice with Nature (especially at higher levels), but you can also just compare the few things you got to choose from when you do and pick the best. With Order it is a lot harder. Either you have to rely on your enemy to bring the prototype, which doesn't happen always, or you have to carry one with yourself. So, the question is:

How to find the best creature to clone in any specific situation?

The rules of thumb and vague guidelines that I follow for now are to aim for a creature with the most Defence, because this means that my summoned creatures would last longer against normal attacks. I also typically aim for high-leveled creatures, because they are typically stronger, plus normally have higher Defence value.

I also typically aim for melee fighters because they are disposable anyway and typically deal more damage than shooters and, what's more important, bind the enemy by engaging them in melee fight.

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.