Typically these will be in your sideboard, but depending on the meta, your deck, and what the card does, sometimes you can be extra hateful and make it work mainboard
The best early play with this planeswalker may be the -2 loyalty ability to double token creation, but the +1 deathtouch-granting ability can also be useful
The useful part is separation
Lifegain decks often struggle to find enough ways to use their life as a resource, and Karlov lets you effectively do so without actually losing the life
It is not however a great starter, you want to take someone from 20 to 0 with just a Thundermaw good luck
, , Sacrifice this creature: You gain 3 life and each opponent loses 3 life