Honestly, all you really need to do to make a mini boss or other “strong” enemy stand out is to give them a significant boost in health. Players tend to conflate how long an enemy survives with its relative strength. The longer the foe lives, the “tougher” it will seem to them – even if it deals zero actual lasting damage to them at all. Since you’re not doing a fully-fledged boss battle (which typically has phases and unique mechanics), you just need to make the “strong” enemies take a notably longer time to kill (and maybe one or two mechanics unique to them just to make them stand out a little). If the player normally cuts down enemies in 1-2 hits, an enemy that lives for 5 or more seconds will be notably different enough to stand out as a mini/mid boss. You don’t actually have to boost their damage output much over a regular enemy, they just have to live longer.
Further reading: [Time to Kill]
The FANTa Project is being rebooted. [What is the FANTa project?]
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