Uhm, I detect high levels of accurate criticism in this thread. Alright, let me try to post my experience with Defensio:
- Definitely the most enjoyable class I've ever played, feels agile, has a lot of air time, can block, can move insanely fast, can apply super armor, can apply damage reduction on teammates, can apply damage debuff on the boss, has self heal moves and its playstyle is frenetic and non-stop.
- Her damage sucks with non L-grade gears (I'm barely able to take one and a half patos oratio with a non elemental build with level 90 gear/ coma+nightmare). This is unconvenient for any party that needs dps.
- Her super armor buff is the most powerful defense buff I've seen and used in this game when applied on DPS classes. The cons to this are pea-sized area and short as heck buff duration (Ruina can do it too, so there goes what could give an advantage over this class). At best, you can cast two BTWs in a 3 second window (one for the boss and one for the team. Notice that when used on a DPS, it can raise its damage by quite a bit). Although being a powerful buff, it's usage is so gimmicky that guardian force (which does a lot less than BTW does) is 100 times more reliable.
- I don't know if her damage reduction stacks with other damage reduction buffs, if it does... that's how much? 20% from guardian + 20% from defensio, I guess. Permanently. What's annoying is that they could had given the damage reduction to taunting blow, but nah, they had to give it to BTW, because why not.
- Her healing is rubbish (but healing after all), but it gives some damage reduction when stepping on it. Its randomness makes it unreliable, like beyond the wall.
- Provoke works perfectly for me, being melee is not bad at all from my point of view (you have to be close to the enemy to give damage reduction anyway, and bouncy skills can be controlled). When doing HC, my defensio did make up for the guardian (who died before because of fire) and could keep the agro on her easily thanks to Repair.
- By using a combination of beyond the wall self+boss, TB and leap over (obviously EX) you get some reliable tankyness (not as good as any other tank).
- A future patch will arrive including a damage buff on Beatdown and leap over (I suspect in their EX versions) and some mechanic changes for ruina. This is not confirmed information, but we'll see.
From my point of view, any party that knows this is a game can take defensio with them to any stage. I myself cleared RDN HC in a random party with my defensio (and by random, I mean people who didnt know each other). While it is true we ended with like 3 minutes of spare time, it was like the most intense run I've done. Most parties in the game can clear RDN HC when missing 3 characters (either because they died or because they are being carried).
From my experience, two spots are guaranteed guardian(main tank)/destroyer(underused)+saint/phys/LF, four spots are usually DPS, the other 2 (you can sacrifice some DPS and get 4 random spots) can be whatever you want and it will not change the outcome of the run at all. Because you know there's only two possible outputs: success or failure.
If you like fast clear times around 20 minutes, dont take the defensio, take an extra full FD character. If you dont mind to take longer to clear, take the defensio. It will not make a difference in an experienced party, if the run fails, it's most likely to someone screwing something up, not to the defensio being a defensio (It's still cute, don't deny it).
To sum up, there is not a specific reason to take a Defensio in your HC party, it is not adviced to use it as a main tank (because it is currently underpowered), but at the same time we all know it is possible to clear without key elements (clears without guardian, without saint or with half the DPS) so... what are you worrying about? You could have a reliable DPS, a carried person, extra healing or a permanent damage reduction on boss + unreliable and occasional butt saver with great survivability (that is the defensio). The decision is only yours.