The fundamental locking method is to match it's 2hs with the same 2h of your own group.
The first time, we let the 2h slip by because of a double DA critical followed by a call Wyvern, but the second time, it used Manafont:
(the following is from the log)
Benediction
Call Wyvern
Perfect Dodge
Invincible
Invincible
Manafont
Chainspell
Benediction
These 2hs were used in that order against Manafont, and it never used Meteor.
After that, it never used Manafont, Chainspell, Benediction or Call Wyvern again.
However, it continued using Invincible, so it is possible that we did not fully lock it.