Most mercs can be very useful if you know what to use them for... as an example, the 30gp Mutant merc group is perfect for Man...
Use them as the expendable meat shield to keep the enemies off of your Longbowmen and rip the guys to shreds...
Tarrasques are, by any normal thought process, a late-game summon. Better investments are the unbreakable wall of Vine Ogres (which you can start getting _very_ early if you have good Nature Magic) and the Elemental Kings/Queens.
Another strategy that is almost fool-proof, imo, is to play any country with good Nature gems and Nature mages and play a N9 or N10 Naga, get Haruspex, then blitz for Summon Lamia. The Naga will get something on the order of 10-12 Lamia per summoning, and they are very solid wall troops against most enemies. An army of 50 of those (4-5 turn investment) will kill most enemy armies up to twice their size (conservatively).
If I have time tonight, I'll spend more time on other strategies. Enjoy,
Wyatt