Targeting Options

What targeting should I put on my monsters for better efficiency?
First off, the default Target Priority should work just fine (furthest in maze). What really makes a difference are the Extra Options available to you. The ones you should consider using the most are "Skills Only on Bosses" and "Skill Support". Keep in mind that "Skill Support" allows towers' skills to override any elemental shields, which is good if you want to set up towers for gold generation or movement impairment.

If you have a gold generating monster (Monsters of the "Fluffy" family, and Conduit), you should consider activating "Skills Only on Bosses" for efficient gold gain (provided you have upgrades to benefit from this). If you have a secondary gold generating monster, you should consider checking the option "Skill Support" and "Focus Attack Bosses", so that you can get even more extra gold efficiently.

If you have Squidler with Anti-Shield Aura, you can place an Air Monster with the Confusion Skill just outside and after (a bit further beyond) Squidler's range, with the "Skill Support" option checked, so that you can confuse any enemy back into Squidler's range, in an attempt to get the resistance removed.

If you have a monster with a movement impairing ability, checking the "Skill Support" option is also a good idea, as you limit the monster to do the purpose it's likely intended for.

Please keep in mind, monsters skills are set to only to activate a maximum of three times per robot. So Confusion and other Movement Control abilities will only activate a max of 3 times.