First off the base sprite forms are in general useful... some are FAR more useful than others...
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The Seraphynx can use it's Beam on chokepoints and turrets, the Heart Attack can improve HP drop rates, the Shield can protect you and allies from damage. For beating the main story the Seraphynx is underwhelming until you gain access to the 2nd skill, then it becomes super useful throughout the game... keeping allies alive and helping you survive dangerous areas...
What to improve?
Heart attack drop rate should vary based on the number of enemies affected.... the fewer the enemies the higher the drop rate...
Shield should change color just like a Knight shield to show when it's close to breaking/ending...
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Both Beam ultimates are useful but the Disintegration ray is far superior.... perhaps replace the Dazzling Ray's Fiend/Undead boost with a Stun status...
This would make the choice between damage or speed control... both powerful options in their own right...
The Violent heart attack is a good improvement over the base while the Iron heart attack is rarely useful.... due in part to how Defense orbs aren't that useful since most attacks in this game (especially in higher levels) ends up taking a very large percentage of your HP... Rather than boosting your armor's defense it should give you a universal status resist...
Both of the Seraphynx shields ultimates are actually useful... the only thing I'd improve is how quickly you can activate them in addition to the color change showing how close the shield is to breaking...
Certainly not the worst thing you've written. I'm glad to hear from you again.
Heart attack drop rate should vary based on the number of enemies affected.
They could do it that way but they chose not to. I'd imagine that they want it to scale like that for a reason- so that you're given incentive to mark and assassinate as many enemies as you can within a short timeframe. This creates high risk and high reward, which is innately thrilling and much of waht Spiral Knights is going for.
replace the Dazzling Ray's Fiend/Undead boost with a Stun status
This would be a nice change.
The Violent heart attack is a good improvement over the base while the Iron heart attack is rarely useful
I don't find either to be particularly impressive but perhaps that's a good thing given how strong heart attacking is. If they were to improve it I wouldn't complain though.
Both of the Seraphynx shields ultimates are actually useful
They are but the valk aura's CTR bonus just sets the weapon to CTR Max, which is basically needed to play anyway at this point... so perhaps it could.. go higher than max? Maybe stack with your existing CTR? Just a thought.
color change showing how close the shield is to breaking
YES. They need this hardcore. I can't tell when my seraph shield is going to break or not!