In order to equip a Tier 2 item, you must purchase a Tier 2 Booster Pack. This is unlocked when you reach Rank 5 in the multiplayer mode. This is all done under the Hero tab on the main menu, and then selecting the "Customize Your Multiplayer Gladiator" option.
Bronze - 600 Gold
Silver - 1200 Gold
Gold - 1800 Gold
Once you have gotten to rank 5 and purchased a Tier 2 booster pack, you must equip one of the unlocked items. Items are sorted by tier when you go to equip them so it is easy to find the right ones. The achievement doesn't actually unlock until you go into a match with the Tier 2 item equipped.
*TA Member ballum87 - Pointed out that it is rank 4 and not rank 10 when Tier 2 is unlocked. I don't know why I put down rank 10 initially...apologies for the confusion!
*TA members boxandthefuzz & ch1cag0steve - Noted that the game has been updated, and tier 2 is now unlocked at rank 5.
*TA Member BigBoomKC - Pointed out an error in one of my paragraphs where I still referred to Tier 2 unlocking at rank 4.