Some techniques (or attacks) have what are called Tech Chains, meaning that your monster starts with a not-so-powerful attack and as it gets stronger it is replaced with a similar but more powerful attack. Often these more powerful techniques require more Guts to use as a trade-off for better a Hit Rate and Damage percentage.
In Monster Rancher 2 tech chains are unlocked once you use the starting attack 50 times. Even if the attack is used on an errantry, and even if you later combine the monster (as long as you keep the Main species the same and select the monster with the unlocked techniques FIRST in the combination process) the counter will not reset.
- For instance, say your Gali starts with "Firewall." Use it 50 times and "Blazewall" becomes available. You use that technique 25 times and then your Gali nears the end of its life, so you combine it with a Monol (Making sure to select your Gali FIRST and the Monol SECOND) and have four possible combination results: Gali, Monol, Galirous, and Ivory Wall. You must choose either Gali or Galirous to retain your unlocked techniques. Now Galirous uses "Blazewall" 25 more times and "Napalm" becomes available.
Not all of the Tech Chains are constant. Some monsters (like Suezo and Tiger) have no tech chains at all. Some have chains that only evolve once, some can evolve up to four times. Some moves only require that you unlock the first evolution in order to get the further ones, and some only require use 30 times instead of 50. Some also depend on the nature of your monster.
A complete explanation of Tech Chains are available HERE.