- if you killed a newbie (bronze star) it's probable you won't be credited with the kill. If the killed mage was gold or silver star and you still didn't get the credit then it's a bug

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- sabotage attacks, Vegetational decay and burn land destroy barriers. You can use any of that to lower someone's bars. The safest way to kill a mage with a spell is to spell him while he's running. It's possible he won't notice in time and take many turns at once (those who like to manacharge 20+ turns in a row

) and boom-he's probably dead or as good as dead. The other possibility is to thoroughly attack him (most often using many counters he gave you) and spell him up when he's on low land and burn-landed. He may not have enough mana to dispell all dangerous enchanments that you put on him.
-dispelling success is somewhat random. But more often than not the target will dispell if he/she uses enough mana for that. Only occasionally the dispell will fail if enough mana is used.