It can fend off attacks from effect monsters and change their battle position to defense position, having 0 DEF. It can be activated outside of the battle phase. However, normal monsters are unaffected.
Ever since Mirror Wall have been buggy, it has been removed from all my decks. Sometimes it just doesn't work, it activates and then just does not half the attack points of the attacking monsters (and no, there weren't cards involved that would negate mirror wall like rose lover for example). It happened to often to lose a duel due to a non-working mirror wall. Wall of Disruption has never failed me like that, so I have those in my deck (which works just as well, is permanent and doesn't cost me lifepoints to keep the effect). Drowning mirror force could argue to be powerfull just like Treacherous trap hole, but direct attacks don't happen so often (usually big clash of monsters and effects) and Treacherous trap hole is good but not worth losing all my trap cards. So it remains Wall of Disruption and Widespread Ruin for me :)
The fact it says the page was last updated December 2016 might have something to do with it. (Evidently it was updated past that date anyway, given the inclusion of Security Orb, but still hints at it being long-abandoned.)
What is the card name that lets you sacrifice a card to increase another cards level?
Best cards now: wall of distruption, floodgate trap hole, super rush headlong, kunai with chain, anti magic arrows together with windstorm of etaqua, enemy controller, mirror wall and sphere kuriboh
Anti meta cards: man eater bug, fenghuang (good backrow removal monster), parallel twister, ancient gear beast (shuts the opponent's backrow down when attacks and negates monster's effects), forbidden chalice and cosmic cyclone
This is my opinion about the cards of the current metagame. What do you think about it?