Good job deciding to try imp game, it`s totally different game then normal/hard
But you really got big leap from warrior to mage/imp, since he is hardest to start with for lack of leadership/big army and strong spells caused with low intellect. How are you with rage spirits ? So my advice is to go with crowd control tactics :
1) get some thorns/royal thorns if available and use their summoning on corpses, they aren`t important for battling enemy units, as in making them lose turns to kill while you dmg with rest of army (meat shield idea). And since they are summoned units cost nothing and you have no losses
2) beholders evil normal, are great for their % to sleep enemy units.
3) griffins/royal snakes are best units to have at early phase of game.great lds/hp ratio and good other stats. Or any other no retaliation combo other snakes with special 1 hex far attack, or marauder/robber with special mace attack.
4) i asked for rage spirits, because on early stage you should have Zerock and Sleem and they have nice controling skills - wall, and gloth armor, use them whenever you can.
5) try leading 1 archmage. Use his magic shield all the time on you`re melee units in middle of enemy army (griffin/snakes). Dont use battle mage skill they arent dmging units, but support so use magic shield and telekinesis to move around enemy away of you`re ranged, or to position you`re tanking units better to block enemy path. Other reason i say only 1 archmage is enough is lower your army total leadership is comparing to enemy, more exp rage spirits get in battles, and 1 archmage have same shock% with his lightning attack as 100 of them and he have lowest dmg of all 4th lvl units, so dont rely on him to kill, but to control battlefield (magic shield, telekinesis, shock%).
6) get somewhere marauders ( only 1 is enough, same reason like archmages), they have 2charges loot corpse and with it I cleared first locations (darion , arlania, verlon forest, swamp) and got to freedom islands with 1,4million gold. 1 thing is i got gift spell and recharged them at least 1 time/battle, but even without it i got 1500-2500 gold from 1 marauder per every battle , just keep him away from enemy units until you kill 2 stacks, run in there loot and run back. If you want ofc you can keep full lsd worth of them and attack enemy also, but i`m suggesting way to save gold and 1 marauder get same gold as 100 with loot skill.
7) spells - dont use spells that give much bonuses to you`re units, since mage have lowest lds, but use spells that weaken enemy : fire arrow, poison skull( they both lower enemy moral if burn/poison effect), trap (among best spells in game), target (if available is great also, cast on you`re strongest unit, and enemy will attack it while you kill them with spells/spirits/other units).
8 ) if you already lost units in battles, i`m guessing no loss isn`t your goal, so why not kill turtle, go to freedom islands and pick whatever isn`t nailed to ground (i dont know how good you are in kiting- luring enemy to move from treasure, and taking it without fighting them) and see what units are available there,maybe you get lucky and pick something useful. And if you do this, finish quests that dont need fighting (
9) you already started imp game on original game, but still in darion locations. I suggest to try this mod, it`s great
http://forum.1cpublishing.eu/showthread.php?t=29080 (maybe not imp start, but try hard diff first, it should be challenging enough). check description and if interested try it out, creator changed and made some things much more interesting you`ll have fun. But warning : it`s harder then regular impossible game (enemy heroes give bonuses to their units, like you with some skills), can have 2xSpell cast/turn like mage with higher magic, some lower your units resistances ( -% to physical or magic . . .) and so on.
I hope i gave you some ideas how to save gold on troops, if any questions post here i`ll try to help. Good luck