Use the field spell "Gaia Power" with this card to ensure him from being destroyed by battle.
If your opponent attempts to swarm the field with strong monsters, try using "Patrician of Darkness" to block their attacks by forcing those monsters to attack him. Alternatively, equip "Raregold Armor" to him.
Equip this card with "Fusion Sword Murasame Blade" to raise its attack to 2200. In addition to the great +800 ATK, your opponent cannot destroy it with effects that destroy Spell Cards, making the only way to destroy "Obnoxious Celtic Guard" by targeting it with a monster-destroying effect.
Using "Comrade Swordsman of Landstar" alongside "Pixie Ring" will raise this card's ATK to 1800 (unable to be destroyed in battle by any monster), plus "Pixie Ring" prevents its ATK-boosting ally from being attacked.
If "Obnoxious Celtic Guard" is in Defense Mode, equip it with the Trap Card "Spikeshield with Chain". While it's in Defense Mode with the Trap in play, it'll add its ATK to its DEF, creating a defense of 2600 points or more. And if you do switch it to Attack Mode, it'll still gain 500 points from the Trap's primary effect, increasing it to 1900 creating a pseudo-wall in both ATK and (especially) DEF.
The above strategy also works really well with "D2 Shield", creating a powerful defense monster, though it won't gain an ATK boost that "Spike Shield" gives as a trade-off, leaving it better suited for staying in Defense Position.