Send Suspicious Bush toward the tower; enemy troops won't attack it, so it reaches the tower easily. The goblins that pop out damage the tower. It's cheap, so cycle decks use it for constant pressure.
Tips
Sending it while your opponent's small spell is out of cycle deals more damage.
Send it alongside another push to split the defense.
Towers and spells can still damage it; it isn't fully safe.
Strengths
Enemy troops don't target it.
Very cheap; builds constant pressure.
Weaknesses
The goblins inside are easily cleared by small spells.