Use this card to force your opponent to initiate their Battle Phase to trigger effects during it (such as those that force them to attack with their monsters by "Evil HERO Malicious Fiend").
You can use this against cards that can only be activated during Main Phase 1, such as "After the Struggle".
One of the biggest virtues of "Timeater" is that without a Main Phase 1, your opponent normally can't Summon and then attack, so you usually have a turn to deal with whatever your opponent Summons in the way of "Timeater".
"Timeater" can also offer a strange warding to monsters that need time to work, like "Stealth Bird" or "Swarm of Locusts", by virtue of keeping your opponent from having usable monsters during their Battle Phase.
Using this card along with "Great Long Nose" makes your opponent virtually skip their turn (by skipping their Battle Phase, the opponent cannot enter their Main Phase 2 thus they go from Standby Phase to the End Phase), but this lasts for only a short period, because "Timeater" needs to destroy a monster for its effect to activate, whereas your opponent cannot Summon or Set due to his turn being skipped. To compensate for this, use cards that make your opponent be in the possession of a monster: "Ojama Trio", "Grinder Golem", "Shien's Spy", a combo of "Imperial Iron Wall" and "Destiny HERO - Departed", or, the easiest choice, "Miracle Flipper". Also take note that "Great Long Nose" is a Spirit monster that needs a tribute to be Summoned, therefore you should have cards that prevent him from returning to the hand like "Spiritual Energy Settle Machine" or "Mirror of Yata". It may be difficult to pull this off, but it's a sure win if these pieces are in place.
In case of the chance your opponent draws a card such as "Effect Veiler" or if they control an attack position monster at the time trying to use "Honest" or "Tragoedia" which may end the lockdown, "Fenrir" could be added into the mix though this would require many monsters to be destroyed by battle at any given time. A more permenant solution to some summons such as "Scrap Golem" could be used or "Black Garden" and "Swap Frog" (of course using all of these cards in the same deck is not very advised.)
Alternatively, you can use this card in conjunction with "Scrap Golem" and "Scrap Searcher" to create an infinite loop (having already either "Timeater"/"Scrap Golem" on them in the field, and using "Scrap Searcher" as the tribute fodder for the remaining one). Special Summon "Scrap Searcher" (by "Scrap Golem's" effect) onto your opponent's field (clearing face-up monsters all the while due to "Scrap Searcher's" second effect), and destroy it with "Timeater" to activate it's effect; repeat the cycle on your next turn since your opponent will not be able to set, play, or summon cards on their Main Phase 1 and 2. This lets you buy time to get a good setup for cards that need either hand advantage or a clean field.
This card is an excellent weapon to use while you have an activated "Helpoemer" in your Graveyard. Your opponent will be forced to enter their Battle Phase, forcing them to discard a card from their hand, even if they do not wish to conduct battle. "Black Garden" will give you a monster to destroy, creating a semi-lock. Add a Spirit monster or something else you can repeatedly summon to force the effect to go off every turn.
Combine this card with "Thunder of Ruler" to skip your opponent's turn. If your opponent cannot enter their Main Phase 1 or Battle Phase, then they cannot enter their Main Phase 2, essentially skipping their turn.