Dragonic Overlord's second effect is not Vanguard-only, and does not require a Kagero Vanguard. You can use this in conjunction with Mr. Invincible and other Nova Grappler units that can unflip damage, in order to be able to use its effect multiple times.
Even so, it is not advisable to splash Overlord in a deck focused mostly on Nova Grapplers, as you run the danger of him depowering due to lack of other Kagero on the field. If this happens, consider Bellicosity Dragon, Bahr or Aermo.
It's also possible to run this unit in a mixed Kagero/Aqua Force deck with Tri-stinger Dragon as your Vanguard. Tri-stinger Dragon will unflip 2 damage almost every turn, which would allow you to use Dragonic Overlord's skill for a net cost of a counter blast 1.
In a well-built deck, it is by no means impossible to use the Overlord's effect twice, or even three times in a row, for an even bigger Power Bonus. With a +10000 or a +15000 bonus to the unit, when boosted by Embodiment of Armor, Bahr it becomes really hard for the opponent to block such an attack. Also notable, the "standing" part of his effect stacks over itself, but since he is already standing, it is completely useless to apply it more than once.
It is beneficial to combine Overlord with a powerful Grade 1 such as Embodiment of Armor, Bahr or Lizard Soldier, Raopia. Attacking a Rear Guard with 9000 Power or less with such a combination will require at least 20000 Shield to block once Overlord's effect is active, and even more if the opponent wants to be safe against a trigger check. Taking advantage of this, especially when you use Overlord's effect as Vanguard, can either ensure your first hit, or at least make your opponent waste enough cards to offset your missed Drive check.
Running Stand triggers with Dragonic Overlord is advantageous. If your Overlord's attack hits while its effect is active, and you check a Stand trigger, you can Stand the Unit behind it for more power during the next attack. Alternatively, if you use Overlord's effect as a Rear Guard and you fail to hit, you still have a chance to Stand him again with the Vanguard's attack and decimate your opponent's field or hand.
If your opponent has 2 grade 2 rear guards, you should wait until one or both is destroyed, because if you do not the opponent can use one of them to intercept so that the Eternal Flame ability is short one attack.
You can also combo this card with Sealed Dragon, Blockade as your vanguard and Dragonic Overlord as your Rear-guard. With Blockade's skill, grade 2 are unable to intercept, therefore Dragonic Overlord will be able to kill two Rear-guard with out worrying if they intercepted. The opponent most surely to not guard from their hands in order to take Blockade's attack, especially if his/her damage is four or more.
If you are not running stand triggers, it may be the best to use Dragonic Overlord to attack after the vanguard, if this unit is used as a rear-guard. All the triggers from your vanguard can be poured into this unit, especially when your attack is negated. But make sure that the opponent did not intercept (if he/she has a Grade 2 in front row) before this unit attacks or Eternal Flame would lose its function.
Break Riding this unit over Dauntless Drive Dragon can help it to stand even after Eternal Flame is blocked, or standing after it attacks the vanguard, especially if you manage to drive check a critical trigger.
You can Break Ride this unit on top of his Break Ride Counterpart to absolutely decimate your opponent's rear guards as they now have to defend against a 26000 unit, potentially more after triggers and boosting, and if Overlord misses the first time, he can use the Break Ride skill to try attack again.