Anyone any advice for a beginner? Picked up this cool game few times but not great at deck making at all
Stick to easy mode and wagers off. wagers on causes difficulty to ramp up much faster and contestants to be eliminated much faster. You'll want wagers off so you can maintain a larger pool of contestants to make it easier to find someone not so mean. Arena Inside at night time has a very important black market merchant that sells mostly very good stuff especially if you got the deck points to fit the cards in.
For deck building my recommendation is focus on a few core cards you'll want to play every turn. Then focus on having cards that either remove themselves or controllably exile non self removing filler like dramatic rescue (which is a starting card). My recommendation is sticking to the 15 card minimum so that it's easier to reach your core deck cards and don't have to put in much draw card and healing filler to help keep yourself alive till your core cards are ready to be played. The idea is to have 10 or less core cards that will be containable in a single hand that you play depending on the situation. The other 5+ cards being draw/healing filler that either self removes or is planned to be destroyed mid match by stuff like dramatic rescue so that it can't get in the way of your 10- core cards being in your hand. Reason the core cards have to be 10 or less is because max hand size is 10. One of the important core cards will be some form of draw card for if you plan to play more than one card in a turn.
Both Gladiator and Parasites (in the standard pack) themes have good means grow height in mid match. I'm not very familiar with gladiator because I dislike the aesthetic so I can't give precise advice there.
In the case of a parasite deck you'll want a card that does hand discarding to discard your parasite cards to get a new copy that you preferably play that turn so that you don't have to keep track of where you are alternating between discarding and playing to make parasites work properly. Parasitic succubus will be a core card since it gives height growth that only gets better the longer the match drags on.
I'm personally partial to fairies myself since it not only has a short match height growth option, but also fairy meteor which permanently grows in damage whenever used. Both gluttonous fairy and fairy meteor remove cards with each use though so you'll need something to add cards to your hand or deck to balance them out.
One of the game's side quests involves opening a bunch of standard boosters anyways. Hitting up the arena inside black market and constructing a decent parasitic deck are probably the best things you can do for initial set up and getting ready for that one odd side quest. If you can manage to pull off a long enough match with parasitic succubus earning tens of thousands of height is entirely possible in one match and it's possible to have 6 digit height. Just having 10k+ height will make it be possible to realistically hunt the legends of a booster and they tend to be really strong cards.
If succubus island is anything like the previous patches odds are you might want a fairy fire/eruption/meteor deck because it's one of the few things that can still potentially deliver a hard punch with severely constrained mana. Also set that island aside for later.... it had some brutal fights that in the earlier versions that were even more lethal than the final boss Valeria.
Some bosses have powerful regen like Valeria. If you don't got a way to potentially ramp yourself up into hundreds of damage in an attack turn you might get stone walled. Although fire fairy fire/eruption starts weak at least it can ramp up. Don't be afraid to experiment with buffs. Some of them are surprisingly powerful.