This card can be used in any deck but works best in a Tachikaze deck. You can build a deck focused around it or use it as a back-up Vanguard in Raptor builds.
You only have to retire a Rearguard when Deathrex's attack hits.
If you do not have any rearguards, you can still attack, the second effect will not activate.
When you have damage available, great to combo with Winged Dragon, Skyptero or Dragon Egg which allow you to Counter Blast (1) and send it to your Hand instead of the Drop Zone.
Raging Dragon, Sparksaurus and Raging Dragon, Blastsaurus are good choices to retire when your Drive Check reveals a Stand Trigger. You can stand the unit in the same column as the Sparksaurus or Blastsaurus that Deathrex retired and can call a new one for another boosted attack.
Boost with Sonic Noa to gain a 23,000 Power attack.
Be aware that him and skyptero are splashable together in any deck that doesn't require to counter blasting as skyptero is a unit that will continue to be reuseable and at no disavtange to the player.
Destruction Dragon, Dark Rex is a great card to use in Deathrex builds. Deathrex can attack for 15000 Power without boosting, enough to force a guard against any card, you can leave the unit behind it standing so that Dark Rex can attack boosted after Superior Riding it. Use this combo when the opponent is at 5 damage and they have no choice but to guard against all your attacks.