Garr

Garr is immune to poisons, so use a [Dense Sharpening Stone] or [Elemental Sharpening Stone] on your off hand (assuming you have Windfury on your main hand).
This fight is one of the longer fights in Molten Core, so you can make make great use of a pet toy here, such as [Ancient Cornerstone Grimoire] or [Arcanite Dragonling] or [Mithril Mechanical Dragonling], etc.
(Do NOT use [Gnomish Battle Chicken] as it is bugged in WoW: Classic.)
Take a look at your clear times. This fight may go long enough to use more than one explosive. If so, use your first one early to ensure you can get a second one off.

This boss comes with friends. Make sure your AoE is available and off cooldown.
Combine Adrenaline Rush and Blade Flurry for maximum DPS.
[Goblin Sapper Charge] off the bat might get you killed, so keep your [Limited Invulnerability Potion], Evasion and Vanish handy.
Garr's friends explode and knockback upon death. Consider your position, you may be able to use it to keep you in the fight!
Just be aware that while you may be able to survive one explosion, you won't survive them all.
Taking risks with your health? Alternative health potions such as [Major Healthstone], [Lily Root], [Whipper Root Tuber], [Crystal Restore], and [Razorlash Root] don't share a cooldown with traditional pots. Double up!