If you haven't chosen yet, consider
Chapuz's tutorial where you start with Darkness to sell most your cards to buy grabbow & rush to grinding ai4
Legacy implements the original's starter decks, which are quite plainly not good. The first thing you'll want to do when starting is prune your deck down to 30 cards in order to increase the damage density. Below spoilers list the starters along with how you should reduce them to 30 cards. openEtG's Legacy mode differs from original by making pillar/pends free, so the starter decks quanta base can also be optimized with pends
Some elements are better off starting against AI2 instead of going straight for AI3. You may prefer to start on AI2 anyways, as even when these decks are marked to face AI3 that's in part due to AI3's much higher rate of card drops, particularly rares which can then be sold
Also given are decks which can go against AI3 reliably with a few purchases from bazaar, & something to build towards to go vs ai4. See Light for a dragontal build; any element can build an ai4 dragontal using that template
Spoiler for Entropy:
030va011030a12c0112d0312e0112f0112g0212h0312i0312k0615g0115i0115j0115k0215m0115o018pj
011030d12c0213u0312i0312k0212h0112g0112d0312e0112f018pt
011030d12c0213u0312i0212h0112g0112d0612e0112f018pt
011030912c0613u0612h0112d0612e0112f018pt
0f13u0612h0412n0421p0121l018pt
Spoiler for Death:
030va011030c15g0115h0415i0115j0215k0315m0215n0215o0115p0115r0318k0118l0118n0118q0118r018pk
011030715g021720215o0415i0215m0215n0115j0215k0115p0115r0115h0118q0118n0118r0118l018pl
buy 4 poison 6 mummy 1 flesh spider
011030615g061720615o0315j0615t0115r0115h018pk
0415g0c1720615t0115h0524t02252018pu
Spoiler for Gravity:
030va011030d18k0218l0218m0218n0118o0118p0218q0218r0218t041bo011bq021bt011c0011c2018pl
011030818k071a60218q0218t0218n0218r0118p0218l0218m0118o018pl
buy 4 sapphire charger 1 acceleration
011030f18k0218q011920218n0618l0218m0118o018pl
0718k0a1a60618l0524t02252018pu
Spoiler for Earth:
030va011030d1bo021bp011bq011br011bs021bt021bu021c0011c1011c2021c3041es011eu011f0011f1011f2018pm
030va011030a1bo021da021bu021bp021c0011bs011bq011c1011br011eu011f1011f2011f0018pn
011030f1bo021bu021bp061c0011bs011bq011c1011br018ps
ai4, but with 6 graboids go for grabbow below asap, this deck is much better with upped shields
0i1bo061c0031bs031c1018ps
Spoiler for Life:
030va011030c1es011et021eu021ev031f0021f1021f2011f3011f4021f6041i0021i1011i4011i6018pn
011030c1es011ge021eu021f2031f0011f4021f6011f3011et021i1011i4011i6018po
011030d1es021ge021eu021f1021f2031f0011f4021f6011f3011et018pp
011030b1es061eu021f1021f2061f0011f4011et018pp
buy 2 heal 3 dragon 2 mitosis
0a1es091ge041f2011f4021f9041et018pn
061es091ge061f0031f60424t02252018pu
Spoiler for Fire:
030va011030c1i0041i1011i2021i3031i4011i5021i6021i8031i9031l4011l6011l7011la018po
011030b1i0041i1021i3031i4021i6011i5021i8011i2011l7011la011l6018pp
011030c1i0031ji031i4021i6061ia021i8011i2018pm
0d1ji061i3021i6032510624t018pu
051i0091ji021i6061ic0624t02252018pu
Spoiler for Water:
030va011030c1l4021l5021l6041l7041l8011l9011la011ld041o8021o9011ob011oc011of018pp
030va01103081l4041l7021l5041l8011la021l6011l9011ld011oc011of011ob018pq
01103071l4071mm041l7011la021l6061le011l9011ld018pp
041l40d1mm061le0524t02252018pu
Spoiler for Light:
030va011030c1o8021o9011oa031ob021oc011od021oe031of011og051rc011rd011re011rh011rk018pq
011030a1o8051pq021oc011od031of031ob011oa011rd011rh011rk011re018pr
011030a1o8041pq021oc061of061ob011oa018pr
061o9061oj031ok011oa0924o0224t03252018pu
061o80b1pq061oa0624t01252018pu
Spoiler for Air:
030va011030c1rc031rd031re011rf011rg031rh011ri011rj031rk041ug011uh021ui011uk018pr
030va01103051rc021su031rd011rg031rh031rk031re011rj011rf011uh011uk021ui018ps
buy 3 firefly queen 2 hope
01103021ok0a1rc051su011rg031rh031rk041rj011rf018pn
021ok011oa041rc0d1su041rk0324t03252018pu
Spoiler for Time:
030va011030e1ug031uh031ui011uj021uk021ul011um021un0421k0221m0121n0121p018ps
040va011030c1ug021uk031ui021ul011uj021un011um0121n0121p018pt
buy 6 ghost 3 reverse time
01103071ug07202051uk021ul011uj061uu011um018ps
061ug0b202021ul041uu0524t02252018pu
Spoiler for Darkness:
030va011030c21k0121l0421m0321n0121o0321p0121q0321s0424o0124p0124q0124r0124s018pt
01103041720821k0421m0321n0121q0221s0221p0121o0121l0124q0124s0124r018pu
buy 6 gargoyle, or 5 dragon
011030c21k0421m0121q0221s0221p0621v0121o0121l018pm
0221k0c2360121q0421t0121p0221l0524t03252018pu
Spoiler for Aether:
030va011030412c0112e0112h0112i0112j0112k0c24o0424p0424q0324r0224s0124t0124v018pu
030va011030112j0112i0112h0112e0824o0326a0424q0124t0224s0324r0124v018pj
011030624o0626a0424q062550124t0224s0324r0124v018pu
0924o0826a042550624t03252018pu
0924o0926a0624t0624v018pu
Cheap decks:
031ok081rc0d1su061rj018pn
0o1l4011l9051lg018pp
0c15g0615o0615r061l5018pp
ai4, this is the best unupped rareless ai4 deck
050va0612j0212h0115t0218q051c0011f7021i6021lf011om021rg0124q018pm
ai4, these cards are often used in FG counters
0c0va041ji031li011ul0121p032510624t018pu
ai4, these cards are often used in FG counters
0c0va0115u0318q011li04202011ut021ul0121p032510624t018pu
060va0612j0115u0318q011li04202061up011ut021ul0121p032510624t018pu