Patch Available - KMST 1.2.406

Guard 6 times in like 5 seconds and cool down for a minute. I don't have max Guardian but at Lv10 it barely ever activates.

False. Twice.
Guardian activates exactly 20% of the time at level 10 (assuming Divine Shield isn't up).
Divine Shield stays up for 60 seconds or six hits with a 30 second cool down.
Please stop openly lying.

Mercedes has a passive skill that recovers 5% HP and MP every 4 seconds starting from Lv1.

That's not tanking related.

60% Evasion, 30% Damage reduction. DERP

Yeah, because 30% = 50%. But go ahead and being undeservably smug.

The instant-kill is moot and extra crit is nice, but like I said, it's only better in a perfect 1 vs. 1 scenario. In places like Zakum where it summons mobs constantly, ACB is more effective.

And it's annoying as pineapple to try to hit the arms with blast. NO RANGE.

The insta-kill is the only problem with Blast in that, as you said, it never helps.
The extra crit is actually rather huge as is the 20% PDR Ignore.
I'm not sure what a 'perfect' 1v1 scenario is, but, if you're attacking mobs at Zakum, you're doing it wrong.

The range of Blast isn't nearly as bad as you're making it out to be. The only arm that you should have any problem hitting is the top right, but you should only be attacking it with Blast after all the others are dead.